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GENETICS 


THE  MACMILLAN  COMPANY 

NEW  YORK   •    BOSTON   •    CHICAGO 
DALLAS   •    SAN    FRANCISCO 

MACMILLAN  &  CO.,  Limited 

LONDON  •    BOMBAY   •    CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  Ltd. 

TORONTO 


GENETICS 

AN  INTRODUCTION  TO  THE   STUDY 

OF  HEREDITY 


BY 


HERBERT  EUGENE  WALTER 

ASSISTANT   PROFESSOR   OF   BIOLOGY 
BROWN   UNIVERSITY 


WITH   72   FIGURES   AND   DIAGRAMS 

11  C.  COLLEGE  OF  A.  &  M.  1. 
Dept.  of  Botany 


THE   MACMILLAN   COMPANY 

1913 

All  rights  reserved 


COPYKIGHT,   1913, 

Bt  the  macmillan  company. 


Set  up  and  electrotyped.     Published  Februaiy,  1913. 


J.  S.  Gushing  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


*u,  /ynr.. 


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*••«».*    if.-.  -     ?? 


■•?.*■ 


THIS   VOLUME 

IS   AFFECTIONATELY   DEDICATED 

TO 

MY  MOTHER 


PREFACE 

The  following  pages  had  their  origin  in  a  course 
of  lectures  upon  Heredity,  given  at  Brown  Univer- 
sity during  the  winter  of  1911-1912,  which  were 
amplified  and  repeated  in  part  the  following  sum- 
mer at  Cold  Spring  Harbor,  Long  Island,  before  the 
biological  summer  school  of  the  Brooklyn  Institute 
of  Arts  and  Sciences. 

An  attempt  has  been  made  to  summarize  for  the 
intelligent,  but  uninitiated,  reader  some  of  the  more 
recent  phases  of  the  questions  of  heredity  which 
are  at  present  agitating  the  biological  world.  It  is 
hoped  that  this  summary  will  not  only  be  of  interest 
to  the  general  reader,  but  that  it  will  also  be  of  serv- 
ice in  college  courses  dealing  with  evolution  and 
heredity. 

The  subject  of  heredity  concerns  every  one,  but 
many  of  those  who  wish  to  become  better  informed 
regarding  it  are  either  too  busily  engaged  or  lack  the 
opportunity  to  study  the  matter  out  for  themselves. 
The  recent  literature  in  this  field  is  already  very 
large,  with  every  indication  that  much  more  is  about 
to  follow,  which  is  a  further  discouragement  to  non- 
technical readers. 

It  may  not  be  a  thankless  task,  therefore,  out  of 
the  jargon  of  many  tongues  to  raise  a  single  voice 

vii 


viii  PREFACE 

which  shall  attempt  to  tell  the  tale  of  heredity. 
There  may  be  a  certain  advantage  in  having  as 
spokesman  one  who  is  not  at  present  immersed  in  the 
arduous  technical  investigations  that  are  making 
the  tale  worth  telling.  The  difficulties  in  under- 
standing this  complicated  subject  may  possibly  be 
realized  better  by  one  who  is  himself  still  struggling 
with  them,  than  by  the  seasoned  expert  who  has 
long  since  forgotten  that  such  difficulties  exist. 

Among  others  I  am  particularly  indebted  to  Dr. 
C.  B.  Davenport  for  many  helpful  suggestions,  to 
my  colleague,  Professor  A.  D.  Mead,  for  reading  the 
manuscript  critically,  to  Dr.  S.  I.  Kornhauser  who 
gave  valuable  aid  in  connection  with  the  chapter 
on  the  Determination  of  Sex,  and  to  my  wife  for 
assistance  in  final  preparation  for  the  press. 

I  wish  to  thank  Professor  H.  S.  Jennings  and  Dr. 
H.  H.  Goddard,  who  have  given  generous  permission 
to  copy  certain  diagrams,  as  well  as  The  Outlook 
Company  and  The  Macmillan  Company  for  the  use 
of  figures  24  and  66,  respectively. 

The  fact  that  all  the  suggestions  which  were  at 
various  times  offered  by  my  kindly  critics  have 
not  been  incorporated  in  the  text,  absolves  them 
from  responsibility  for  whatever  remains. 

H.  E.  W. 

Providence,  R.  I., 
September,  1912. 


CONTENTS 


CHAPTER 

I.     Introduction. 

1.  The  triangle  of  life  '  . 

2.  A  definition  of  heredity 

3.  The  maintenance  of  life 

4.  Somatoplasm  and  germplasm 

II.     The  Carriers  of  the  Heritage. 

1.  Introduction 

2.  The  cell  theory  . 

3.  A  typical  cell 

4.  Mitosis       .... 

5.  Amitosis     .... 

6.  Sexual  reproduction    . 

7.  Maturation 

8.  Fertilization 

9.  Parthenogenesis 

10.  The  hereditary  bridge 

11.  The  determiners  of  heredity 

12.  The  chromosome  theory 

13.  The  enzyme  theory  of  heredity 

14.  Conclusion 


III.     Variation. 


1.  The  most  invariable  thing  in  nature 

2.  The  universality  of  variation 

3.  The  kinds  of  variation  with  respect  to 

a.   Nature 

h.    Duplication 

c.  Utility 

d.  Direction  in  evolution 

e.  Source 
/.    Normality 
g.    Degree  of  continuity  . 
h.    Character  . 
i.    Relation  to  an  average  standard 
j.    Heritability        .         .         .         . 


their 


PAGE 
1 

4 

5 

10 

14 
14 
15 

18 
20 
20 
22 
24 
26 
27 
28 
29 
33 
35 

36 

37 

38 
39 
39 
39 
40 
40 
40 
41 
41 
41 


IX 


CONTENTS 


CHAPTER  PAGE 

4.  Methods  of  studying  variation 42 

5.  Biometry 42 

6.  Fluctuating  variation 43 

7.  The  interpretation  of  variation  curves         ...  47 

a.  Relative  variability    ......  47 

b.  Bimodal  curves           ......  47 

c.  Skew  polygons  .......  50 

8.  Graduated  and  integral  variations       ....  52 

9.  The  causes  of  variation       ......  52 

a.  Darwin's  attitude       ......  52 

b.  Lamarck's  attitude     ......  53 

c.  Weismann's  attitude           .         ....  55 

d.  Bateson's  attitude      ......  55 

IV.     Mutation. 

1.  The  mutation  theory  .......  56 

2.  Mutation  and  fluctuation  ......  57 

3.  Freaks 58 

4.  Kinds  of  mutation      .......  59 

5.  Species  and  varieties  .......  60 

6.  Plant  mutations  found  in  nature         ....  63 

7.  Lamarck's  evening  primrose        .....  64 

8.  Some  mutations  among  animals           ....  67 

9.  Possible  explanations  of  mutation       ....  69 
10.   A  summary  of  the  mutation  theory     ....  72 

V.   The  Inheritance  of  Acquired  Characters. 

1.  Summary  of  preceding  chapters  .....  74 

2.  The  bearing  of  this  chapter  upon  genetics  ...  75 

3.  The  importance  of  the  question  .....  75 

4.  An  historical  sketch  of  opinion   .....  76 

5.  Confusion  in  definitions       ......  77 

6.  Weismann's  conception  of  acquired  characters     .         .  78 

7.  The  distinction  between  germinal  and  somatic  charac- 

ters           79 

8.  What  variations  reappear  ?  .         .         .         .         .80 

9.  What  may  cause  germplasm  to  vary  or   to   acquire 

new  characters  ?      .         .         .         .         .         .81 

10.   Weismann's  reasons  for  doubting  the   inheritance  of 

acquired  characters         .....  84 


CONTENTS 


XI 


CHAPTER 


11.  No  known  mechanism  for  impressing  germplasm  with 

somatic  characters         .         .         .         .         . 

12.  Evidence  for  the  inheritance  of  acquired  characters 

inconclusive  . 

a.  Mutilations 

b.  Environmental  effects 

c.  The  effects  of  use  or  disuse 

d.  Disease  transmission 

13.  The  germplasm  theory  sufficient  to 

facts  of  heredity    . 

14.  The  opposition  to  Weismann     . 

15.  Conclusion         .... 


account  for  the 


PAGE 

84 

86 
87 
88 
91 
92 

94 
95 
96 


VI.     The  Pure  Line. 

1.  The  unit  character  method  of  attack 

2.  Galton's  law  of  regression 

3.  The  idea  of  the  pure  line 

4.  Johanssen's  nineteen  beans 

5.  Cases  similar  to  Johanssen's  pure  lines 

6.  Tower's  potato-beetles 

7.  Jennings'  work  on  Paramecium 

8.  Phenotypical  and  genotypical  distinctions 

9.  The  distinction  between  a  population  and  a  pure  line 
10.  Pure  lines  and  natural  selection  .         .         .         . 

VII.     Segregation  and  Dominance. 

1.  Methods  of  studying  heredity    . 

2.  The  melting-pot  of  cross-breeding 

a.  Blending  inheritance 

b.  Alternative  inheritance     . 

c.  Particulate  inheritance 

3.  Johann  Gregor  Mendel 

4.  Mendel's  experiments  on  garden  peas 

5.  Some  further  instances  of  Mendel's  law 

6.  The  principle  of  segregation 

7.  Homozygotes  and  heterozygotes 

8.  The  identification  of  a  heterozygote  . 

9.  The  presence  and  absence  hypothesis 
10.   Dihybrids 


97 
98 
102 
103 
107 
108 
110 
113 
115 
118 


120 
120 
121 
121 
121 
123 
124 
128 
130 
131 
132 
132 
133 


Xll 


CONTENTS 


CHAPTER  PAGE 

11.  The  case  of  the  trihybrid 140 

12.  Conclusion 143 

13.  Summary 144 


ones 


new 


Vin.    Reversion  to  Old  Types  and  the  Making  of  New 

Ones. 


1.  The  distinction  between  reversion  and  atavism 

2.  False  reversion  ..... 

a.   Arrested  development 

6.    Vestigial  structures .... 

c.  Acquired  characters  resembling  ancestral 

d.  Convergent  variation 

e.  Regression       ..... 

3.  Explanation  of  reversion  .... 

4.  Some  methods  of  improving  old  and  establishing 

types 

The  method  of  Hallet 
The  method  of  Rimpau  .         . 
The  method  of  de  Vries   . 
The  method  of  Vilmorin  . 
The  method  of  Johanssen 
The  method  of  Burbank 
The  method  of  Mendel    . 

5.  The  factor  hypothesis       .... 

a.   Bateson's  sweet  peas 

Castle's  agouti  guinea-pigs 

Cuenot's  spotted  mice 

Miss  Durham's  intensified  mice 

Castle's  brown-eyed,  yellow  guinea-pigs 

6.  Rabbit  phenotypes  ..... 

7.  The  kinds  of  gray  rabbits 

8.  Conclusion       ...... 


a. 
b. 
c. 
d. 
e. 

/. 
9- 


b. 
c. 
d. 
e. 


146 
149 
149 
149 
150 
150 
151 
151 

152 
152 
153 
154 
155 
155 
156 
157 
159 
160 
163 
164 
165 
166 
169 
171 
173 


IX.     Blending  Inheritance. 

1.  The  relative  value  of  dominance  and  segregation      .  174 

2.  Imperfect  dominance        ......  175 

3.  Delayed  dominance  ......  177 

4.  "  Reversed "  dominance 178 


CONTENTS 


Xlll 


CHAPTER 


5.  Potency  . 

a.  Total  potency 

b.  Partial  potency 

c.  Failure  of  potency 

6.  Blending  inheritance 

7.  The  case  of  rabbit  ears 

8.  The  Nilsson-Ehle  discovery 

9.  The  application  of  Nilsson-Ehle's  explanation  to  the 

case  of  rabbit  ear-length 
10.   Human  skin  color     . 


X.     The  Determination  of  Sex. 

1.  Speculations,  ancient  and  modern 

2.  The  nutrition  theory 

3.  The  statistical  study  of  sex 

4.  Monochorial  twins    . 

5.  Selectiv^e  fertilization 

6.  The  neo-Mendelian  theory  of  sex 

a.  Microscopical  evidence 

1.  The  "x"  chromosome 

2.  Various  forms  of  x  chromosomes 

3.  Sex  chromosomes  in  parthenogenesis 

b.  Castration  and  regeneration  experiments 

c.  Sex-limited  inheritance 

1.  Color-blindness 

2.  The  English  currant-moth 

d.  Behavior  of  hermaphrodites  in  heredity 


7.    Conclusion 


XI.    The  Application  to  Man. 


1. 
2. 
3. 


PAGE 

179 
179 
180 
180 
182 
183 
186 

193 
196 


197 
198 
200 
201 
202 
205 
207 
207 
208 
210 
210 
213 
214 
216 
220 
222 


5. 
6. 


The  application  of  genetics  to  man   ....  224 

Modifying  factors  in  the  case  of  man         .         .         .  225 

Experiments  in  human  heredity         ....  227 

a.   The  Jukes 227 

6.    The  descendants  of  Jonathan  Edwards     .         .  228 

c.   The  Kallikak  family 229 

Moral  and  mental  characters  behave  like   physical 

ones 230 

The  character  of  human  traits  .....  231 

Hereditarv  defects    .         .         .         .         .         .         .  232 


XIV 


CONTENTS 


CHAPTEB 

7.  The  control  of  defects       .... 

8.  Inbreeding 

9.  Experiments  to  test  the  effects  of  inbreeding 

10.  The  influence  of  proximity 

11.  Inbreeding  in  the  Ught  of  Mendelism 

XII.     Human  Conservation. 

1.  How  mankind  may  be  improved 

2.  More  facts  needed    ...... 

3.  More  appHcation  of  what  we  know  necessary    . 

4.  The  restriction  of  undesirable  germplasm  through 

a.   The  control  of  immigration 

More  discriminating  marriage  laws 
An  educated  sentiment     . 
The  segregation  of  defectives    . 
Drastic  measures 
The  conservation  of  desirable  germplasm 
By  subsidizing  the  fit 
By  enlarging  individual  opportunity 
By  preventing  germinal  waste  . 

1.  Preventable  death    . 

2.  Social  hindrances 
6.   Who  shall  sit  in  judgment  ? 


h. 
c. 
d. 
e. 


a. 
b. 
c. 


XIII,    Bibliography 


XIV.  Index 


PAGE 

235 
238 
240 
241 
242 


244 
245 
247 

248 
250 
251 
252 
254 
255 
256 
258 
258 
258 
259 
260 

263 

265 


GENETICS 


GENETICS 

CHAPTER  I 

INTRODUCTION 
1.   The  Triangle  of  Life 

Within  a  generation  the  center  of  biological  inter- 
est has  gradually  been  swinging  from  the  origin  of 
species  to  the  origin  of  the  individual.  The  nine- 
teenth century  was  Darwin's  century.  His  monu- 
mental work  "On  the  Origin  of  Species  by  Means  of 
Natural  Selection,"  which  appeared  in  1859,  not  only 
dominated  the  biological  sciences  but  also  influenced 
profoundly  many  other  realms  of  thought,  partic- 
ularly those  of  philosophy  and  theology. 

Now,  at  the  beginning  of  the  twentieth  century, 
a  particular  emphasis  is  being  laid  upon  the  study 
of  heredity.  The  interpretation  of  investigations 
along  this  line  of  research  has  been  made  possible 
through  the  cumulative  discoveries  of  many  things 
that  were  not  known  in  Darwin's  day.  Trained 
students  have  been  patiently  and  persistently  bend- 
ing over  improved  microscopes,  untangling  the 
mysteries  of  the  cell,  while  an  increasing  host  of  in- 
vestigators, inspired  by  the  Austrian  monk  Mendel, 
have  been  industriously  devoting   their  energies  to 


nOF&RTY  UBRARY 

n,  C.  State  C»Be|i 


2 


GENETICS 


breeding  animals  and  plants  with  an  insight  denied 
to  breeders  of  preceding  centuries. 

The  study  of  the  origin  of  the  individual,  which 
has  grown  out  of  the  more  general  consideration  of 
the  origin  of  species,  forms  the  subject-matter  of 
heredity,  or,  to  use  the  more  definitive  word  of  Bate- 
son,  of  genetics. 

It   is   not   with   the   individual   as   a   whole   that 


HER!       T     A      G     E 

Fig.  1.  —  The  triangle  of  life. 

genetics  is  chiefly  concerned,  but  rather  with  char- 
acteristics of  the  individual. 

Three  factors  determine  the  characteristics  of  an 
individual,  namely,  environment,  training,  and  heri- 
tage as  expressed  diagrammatically  in  Figure  1.  It 
may  indeed  be  said  that  an  individual  is  the  result 
of  the  interaction  of  these  three  factors  since  he 
may  be  modified  by  changing  any  one  of  them. 
Although  no  one  factor  can  possibly  be  omitted,  the 
student  of  genetics  places  the  emphasis  upon  heritage 
as  the  factor  of  greatest  importance.     Heritage,  or 


INTRODUCTION  3 

*' blood,"  expresses  the  innate  equipment  of  the  indi- 
vidual. It  is  what  he  actually  is  even  before  birth. 
It  is  his  nature.  It  is  what  determines  whether  he 
shall  be  a  beast  or  a  man.  Consequently  in  the 
diagram  (Fig.  1),  the  triangle  of  life  is  represented 
as  resting  solidly  upon  the  side  marked  "heritage" 
for  its  foundation. 

Environment  and  training,  although  indispensable, 
are  both  factors  which  are  subsequent  and  secondary. 
Environment  is  what  the  individual  has,  for  example, 
housing,  food,  friends  and  enemies,  surrounding  aids 
which  may  help  him  and  obstacles  which  he  must 
overcome.  It  is  the  particular  world  into  which  he 
comes,  the  measure  of  opportunity  given  to  his 
particular  heritage. 

Training,  or  education,  on  the  other  hand,  repre- 
sents what  the  individual  does  with  his  heritage  and 
environment.  Lacking  a  suitable  environment  a 
good  heritage  may  come  to  naught  like  good  seed 
sown  upon  stony  ground,  but  it  is  nevertheless  true 
that  the  best  environment  cannot  make  up  for 
defective  heritage  or  develop  wheat  from  tares. 

The  absence  of  sufficient  training  or  exercise  even 
when  the  environment  is  suitable  and  the  endowment 
of  inheritance  is  ample  will  result  in  an  individual 
who  falls  short  of  his  possibilities,  while  no  amount 
of  education  can  develop  a  man  out  of  the  heritage 
of  a  beast.  Consequently  the  biologist  holds  that, 
although  what  an  individual  has  and  does  is  un- 
questionably of  great  importance,  particularly  to  the 
individual  himself,  what  he  is,  is  far  more  important 


4  GENETICS 

in  the  long  run.  Improved  environment  and  educa- 
tion may  better  the  generation  already  born.  Im- 
proved blood  will  better  every  generation  to  come. 

What,  then,  is  this  "blood"  or  heritage?  Ex- 
actly what  is  meant  by  heredity  ? 

2.   A  Definition  of  Heredity 

Professor  Castle,  in  his  recent  book  on  "  Heredity 
in  Relation  to  Evolution  and  Animal  Breeding,"  has 
defined  heredity  as  "  organic  resemblance  based  on 
descent."  The  son  resembles  his  father  because  he 
is  a  "  chip  off  the  old  block."  It  would  be  still 
nearer  the  truth  to  say  that  the  son  resembles  his 
father  because  they  are  both  chips  from  the  same  block, 
since  the  actual  characters  of  parents  are  never  trans- 
mitted to  their  offspring  in  the  same  way  that  real 
estate  or  personal  property  is  passed  on  from  one 
generation  to  another.  When  the  son  is  said  to  have 
his  father's  hair  and  his  mother's  complexion  it 
does  not  mean  that  paternal  baldness  and  a  vanish- 
ing maternal  complexion  are  the  inevitable  conse- 
quences. 

Biological  inheritance  is  more  comparable  to  the 
handing  dow^n  from  father  to  son  of  some  valuable 
patent  right  or  manufacturing  plant  by  means  of 
which  the  son,  in  due  course  of  time,  may  develop  an 
independent  fortune  of  his  own,  resembling  in  charac- 
ter and  extent  the  parental  fortune  similarly  derived 
although  not  identical  with  it. 

So   it   comes   about   that   "organic   resemblance" 


INTRODUCTION  5 

between  father  and  son,  as  well  as  that  which  often 
appears  between  nephew  and  uncle  or  even  more 
remote  relatives,  is  due  not  to  a  direct  entail  of  the 
characteristics  in  question,  but  to  the  fact  that  the 
characteristics  are  "based  on  descent"  from  a 
common  source.  In  other  words,  an  *' hereditary 
character"  of  any  kind  is  not  an  entity  or  unit  which 
is  handed  down  from  generation  to  generation,  but 
is  rather  a  method  of  reaction  of  the  organism  to  the 
constellation  of  external  environmental  factors  under 
which  the  organism  lives. 

To  unravel  the  golden  threads  of  inheritance 
which  have  bound  us  all  together  in  the  past,  as 
well  as  to  learn  how  to  weave  upon  the  loom  of  the 
future,  not  only  those  old  patterns  in  plants  and 
animals  and  men  which  have  already  proven  worth 
while,  but  also  to  create  new  organic  designs  of  an 
excellence  hitherto  impossible  or  undreamed  of,  is 
the  inspiring  task  before  the  geneticist  to-day. 

3.   The  Maintenance  of  Life 

So  far  as  we  know,  every  living  thing  on  the  earth 
to-day  has  arisen  from  some  preceding  form  of  life. 

How  the  first  spark  of  life  began  will  probably 
always  be  a  matter  of  pure  speculation.  Whether 
the  beginnings  of  what  is  called  life  came  through 
space  from  other  worlds  on  meteoric  wings,  as  Lord 
Kelvin  has  suggested  ;  whether  it  was  spontaneously 
generated  on  the  spot  out  of  lifeless  components ; 
or  whether  life  itself  was  the  original  condition  of 


6  GENETICS 

matter,  and  the  one  thing  that  must  be  explained  is 
not  the  origin  of  Hfe,  but  of  the  non-Hving,  no  one 
can  say.  Leaving  aside  the  first  speculation  as  un- 
tenable and  the  third  as  irrational,  since  it  jars  so 
sadly  with  what  astronomers  tell  us  of  the  probable 
evolution  of  worlds,  the  theory  of  spontaneous  gener- 
ation seems  to  be  the  last  resort  to  which  to  turn. 

In  prescientific  days  this  idea  of  spontaneous 
generation  presented  no  great  difficulties  to  our 
imaginative  and  credulous  ancestors.  John  Milton, 
with  the  assurance  of  an  eye-witness,  thus  described 
the  inorganic  origin  of  a  lion  :  — 

*'  The  grassy  clods  now  calved  ;  now  half  appears 
The  tawny  lion,  pawing  to  get  free 
His  hinder  parts  —  then  springs  as  broke  from  bonds, 
And  rampant  shakes  his  brindled  mane." 

("  Paradise  Lost,"  Book  VII,  line  543.) 

Ovid  also  in  his  ''Metamorphoses,"  not  to  mention  a 
more  familiar  instance,  easily  succeeded  in  creating 
mankind  from  the  humble  stones  tossed  by  the 
juggling  hands  of  Deucalion  and  Pyrrha. 

Although  under  former  conditions  on  the  earth 
it  might  have  been  possible  for  life  to  have  originated 
spontaneously,  and  although  it  may  yet  be  possible 
to  produce  life  from  inorganic  materials  in  the  labora- 
tory or  elsewhere,  the  exhaustive  work  of  Pasteur, 
Tyndall  and  others  effectually  demonstrated  a  genera- 
tion ago  that  to-day  living  matter  always  arises  from 
preceding  living  matter  and  this  conclusion  is  gener- 
ally accepted  as  an  axiom  in  genetics. 


INTRODUCTION  7 

There  are  various  methods  of  producing  more  life, 
given  a  nest-egg  of  living  substance  with  which  to 
start.  Any  organism,  whether  plant  or  animal,  is 
continually  transforming  inorganic  and  dead  material 
into  living  tissue.  Through  the  process  of  repair, 
for  example,  an  injury  to  a  form  as  highly  developed 
even  as  man  is  frequently  made  good,  if  it  is  not  too 
extensive,  as  in  the  case  of  a  skin  wound. 

When  the  intake  of  non-living  material  is  in  excess 
of  the  outgo,  growth  results,  w4th  the  consequence 
that  more  living  substance  is  built  up  than  existed 
before.  Thus  a  fragment  of  a  living  sponge  or  a 
piece  of  a  begonia  leaf  are  each  sufficient  to  restore  a 
duplicate  of  the  original  organism. 

A  process  similar  to  the  repair  of  the  begonia  leaf 
is  that  employed  so  effectively  in  the  great  groups  of 
the  one-celled  animals  and  plants,  the  Protozoa  and 
Protophyta,  by  means  of  which  their  numbers  are 
maintained.  These  one-celled  organisms  multiply 
by  fission,  that  is,  by  equal  division  into  halves,  and 
each  half  then  grows  to  the  size  of  the  parent  organism 
from  which  it  sprang.  When  two  daughter  pro- 
tozoans are  thus  formed,  they  are  essentially  orphans 
because  they  have  no  parents,  alive  or  dead.  The 
parental  substance  in  such  a  process,  along  with  the 
regulating  power  necessary  to  reorganization,  goes 
over  bodily  into  the  next  generation  in  the  forma- 
tion of  the  daughter-cells,  leaving  usually  no  re- 
mains whatever  behind.  In  primitive  forms  of  this 
description,  continuous  life  is  the  natural  order, 
and  death,  when  it  does  occur,  is,  as  Weismann  has 


8  GENETICS 

pointed  out,  accidental  and  quite  outside  the  plan  of 
nature. 

In  these  cases  it  is  easy  to  see  the  reason  for  "or- 
ganic resemblance"  between  successive  generations. 
Parent  and  offspring  are  successive  manifestations 
of  the  same  thing,  just  as  the  begonia  plant,  restored 
from  a  fragment  of  a  begonia  leaf,  is  simply  an  ex- 
tension of  the  original  plant. 

Many  modifications  of  the  process  of  multiplica- 
tion by  fission  occur,  all  of  them,  however,  agreeing 
in  the  fundamental  principle  that  the  progeny  re- 
semble the  parents  because  they  are  pieces  of  the 
parents. 

Thus  the  greening  apple  maintains  its  individuality 
although  coming  from  thousands  of  different  trees, 
because  all  of  these  trees  through  the  asexual  process 
of  grafting  are  continuations  of  the  one  original 
Rhode  Island  greening  tree  grown  by  Dr.  Solomon 
Drowne  in  the  town  of  Foster,  nearly  a  century  ago. 

Again,  certain  fresh-water  sponges  and  bryozoans, 
quite  unlike  any  of  their  marine  relatives,  keep  a 
foothold  from  year  to  year  within  their  particular 
shallow  fresh-water  habitats  by  isolating  well  pro- 
tected fragments  of  themselves  in  the  form  of 
gemmules  and  statohlasts.  These  structures  may 
drop  to  the  muddy  bottom  and  live  in  a  dormant 
condition  throughout  the  icy  winter  when  it  would 
not  be  possible  for  the  entire  organism  to  survive 
near  the  surface. 

In  order  to  meet  the  conditions  imposed  by  winter, 
however,  these  fragments  have  become  so  modified 


INTRODUCTION  9 

as  temporarily  to  lose  their  likeness  to  the  parent 
generation,  although  readily  regaining  that  likeness 
when  springtime  brings  the  opportunity.  The  unity 
of  two  succeeding  generations,  although  interrupted 
by  the  temporary  interposition  of  something  ap- 
parently different  in  the  form  of  gemmules  or  stato- 
blasts,  is  thus  essentially  maintained.  The  bryozoan 
colonies  of  two  successive  seasons  in  a  fresh-water 
pond  may  be  regarded  as  parts  of  the  same  identical 
colony,  since  they  present  an  "organic  resemblance 
based  on  descent,"  although  the  sole  representatives 
of  the  parent  colony  during  midwinter  may  be  the 
sparks  of  life  locked  up  within  the  statoblasts  buried 
in  the  mud. 

Similarly,  the  asexual  spores  of  many  plants,  such 
as  molds,  mosses  and  ferns,  may  be  regarded  as 
gemmules  reduced  to  the  lowest  terms,  namely,  to 
single  cells.  As  in  the  preceding  cases  so  in  this 
instance  the  resemblance  of  the  offspring  which  may 
arise  from  these  spores,  to  the  parents  which  pro- 
duced them,  is  due  to  the  essential  material  identity 
of  two  generations. 

These  illustrations  of  heredity  in  its  simplest  mani- 
festations give  the  key  to  "organic  resemblance" 
higher  up  in  the  scale.  Sexual  reproduction  is  no 
less  plainly  the  direct  continuation  of  life  though  in 
this  instance  tivo  sporelike  fragments  out  of  one 
generation  contribute  to  form  the  new  individual  of 
the  next  generation  instead  of  one  fragment.  In  all 
cases  there  is  a  material  continuity  between  succeeding 
generations.     Offspring  become  thus  an  extension  of 


10  GENETICS 

a  single  parent  or  of  two  parents,  while  heredity  is 
simply  "organic  resemblance  based  on  descent." 

4.   Somatoplasm  and  Germplasm 

In  forms  that  reproduce  sexually  there  theoretically 
occurs  a  differentiation  of  the  body  substance  into 
what  Weismann  terms  somatoplasm  and  germplasm. 

The  somatoplasm  includes  the  body  tissues,  that 
is,  the  bulk  of  the  individual,  which  is  fated  in  the 
course  of  events  to  complete  a  life-cycle  and  die. 
The  germplasm,  on  the  contrary,  is  the  immortal 
fragment  freighted  with  the  power  to  duplicate  the 
whole  organism  and  which,  barring  accident,  is  des- 
tined to  live  on  and  give  rise  to  new  individuals. 

The  germplasm  thus  carries  potencies  for  develop- 
ing both  germplasm  and  somatoplasm,  while  the 
somatoplasm,  according  to  this  conception,  has  only 
the  power  to  reproduce  more  of  its  own  kind.  More- 
over, the  germplasm  is  not  formed  afresh  in  each  gen- 
eration, neither  does  it  arise  anew  when  the  individual 
reaches  sexual  maturity,  but  it  is  a  continuous  sub- 
stance present  from  the  beginning.  Although  this 
theory  of  the  continuity  of  the  germplasm  has  been 
actually  demonstrated  in  comparatively  few  instances, 
all  the  facts  we  know  concerning  the  behavior  of  the 
germinal  substance  are  consistent  with  it. 

In  many  of  the  Protozoa  the  entire  organism  is 
possibly  comparable  to  germplasm,  but  in  all  forms 
of  life  that  are  compounded  of  several  cells  the  germ- 
plasm is  probably  set  aside  early  in  the  development 


INTRODUCTION 


11 


of  the  individual,  and  this  remains  undifferentiated, 
or  in  reserve,  like  a  savings-bank  account  put  by 
for  a  rainy  day, 
while  the  somato- 
plasm is  expended 
in  the  immediate 
demands  of  the 
tissues  that  make 
up  the  individual. 
In  one  instance  at 
least,  that  of  the 
nematode  worm 
Ascaris,  as  con- 
firmed by  Boveri, 
this  splitting  off 
or  isolation  of  the 
germplasm  occurs 
with  the  very  first 
cleavage  of  the 
fertilized  egg  into 
the  two-celled 
stage,  when  one 
of  the  two  cells 
forms  the  future 
germplasm,  while 
the  other  differen- 
tiates by  succes- 
sive divisions  into 
the  animal  itself. 

Thus  there  results  a  continuous  stream  of  germ- 
plasm,   receiving    contributions    from    other    germ- 


Germplasm  V  Somatoplasm 

Fig.  2. — Scheme  to  illustrate  the  continuity  of 
the  germplasm.  Each  triangle  represents  an 
individual  made  up  of  germplasm  (dotted)  and 
somatoplasm  (undotted) .  The  beginning  of  the 
life  cycle  of  each  individual  is  represented  at 
the  apex  of  the  triangle  where  germplasm  and 
somatoplasm  are  both  present.  As  the  indi- 
\ddual  develops  each  of  these  component  parts 
increases.  In  sexual  reproduction  the  germ- 
plasms  of  two  individuals  unite  into  a  common 
stream  to  which  the  somatoplasm  makes  no 
contribution.  The  continuity  of  the  germ- 
plasm is  shown  by  the  heavy  broken  line  into 
which  run  collateral  contributions  from  suc- 
cessive sexual  reproductions. 


12  GENETICS 

plasmal  streams  at  the  time  of  sexual  reproduction, 
as  shown  diagrammatically  in  Figure  2,  in  which 
individuals  are  represented  by  triangles.  From 
this  continuous  stream  of  germplasm  there  split  off 
at  successive  intervals  complexes  of  somatoplasm,  or 
*' individuals,"  which  go  so  far  on  the  road  of  speciali- 
zation into  tissues  that  the  power  to  be  "  born 
again "  is  lost,  and  so  after  a  time  they  die,  while 
the  germplasm,  held  in  reserve,  lives  on. 

This  is  what  is  meant  by  saying  that  a  father  and 
son  owe  their  mutual  resemblance  to  the  fact  that  they 
are  chips  off  the  same  block  rather  than  by  saying 
that  the  son  is  a  chip  off  the  paternal  block.  Both 
somatoplasms  are  developments  at  different  inter- 
vals from  the  same  continuous  stream  of  germplasm 
instead  of  one  somatoplasm  being  derived  from  a 
preceding  one.  As  a  matter  of  fact  the  germplasm 
from  which  the  son  arises  is  modified  by  the  addition 
of  a  maternal  contribution,  so  that  father  and  son  in 
reality  hold  the  same  relation  to  each  other  that  half- 
brothers  do. 

From  the  point  of  view  of  genetics,  then,  the  real 
mission  of  the  somatoplasm,  which  is  so  marvelously 
differentiated  into  all  the  various  forms  that  we  call 
animals  and  plants,  is  simply  to  serve  as  a  temporary 
domicile  for  the  immortal  germplasm.  Thus  the 
parent  becomes  as  it  were  the  "trustee  of  the  germ- 
plasm," but  not  the  producer  of  the  offspring. 

In  the  light  of  these  preliminary  explanations  it  is 
plain  that  the  hopeful  point  of  attack  in  the  science 
of  genetics  must  inevitably  be  the  germplasm  which 


INTRODUCTION  13 

is  the  source,  or  point  of  departure,  in  the  formation 
of  each  new  individual,  rather  than  the  somatoplasm, 
which  represents  the  end  stages  of  the  hereditary 
processes. 

This  has  not  been  the  method  of  the  past.  The 
resemblances  of  the  visible  father  and  son  have 
usually  been  traced  instead  of  the  character  of  their 
unseen  germplasms.  By  following  this  old  method, 
investigators  have  often  been  misled  because  the 
visible  or  apparent  is  not  always  the  true  index  of 
what  lies  behind  it.  A  gray  and  a  white  rabbit,  for 
example,  may  produce  some  offspring  that  are 
entirely  black  just  as  two  white-flowering  sweet  peas 
when  crossed  may  sometimes  produce  purple  blos- 
soms. Consequently  it  is  a  great  fallacy  to  affirm 
that  in  heredity  *'like  produces  like,"  since  the  op- 
posite is  quite  often  the  case. 

The  new  heredity,  embodied  in  the  science  of 
genetics,  attempts  to  go  deeper  than  the  surface 
appearance  of  the  somatoplasm.  It  aims  to  get  at 
the  source  or  origin  of  organisms,  that  is,  the  germ- 
plasm  which  is  the  only  connecting  thread  between 
succeeding  generations  of  living  forms.  It  is  con- 
cerned not  so  much  with  somatoplasm,  which  repre- 
sents what  the  germplasm  has  done  in  the  past,  as 
with  the  germplasm  and  what  it  can  do  in  the  future. 


CHAPTER  n 

THE  CARRIERS    OF    THE   HERITAGE 
1.   Introduction 

Heredity,  as  has  been  shown  in  the  introductory 
chapter,  is  essentially  a  matter  of  continuity  between 
succeeding  generations  of  living  organisms.  This 
continuity  may  be  direct,  as  when  a  mother  protozoan 
divides  into  two  daughters,  or  it  may  be  indirect,  as 
illustrated  by  the  relationship  of  a  father  and  son, 
an  uncle  and  nephew,  or  any  other  relatives  of  varying 
degrees  of  kinship  which,  taken  singly  or  collectively, 
are  somatoplasms  derived  from  a  common  stream  of 
germplasm. 

It  is  the  purpose  of  the  present  chapter  to  consider 
this  material  continuity  between  succeeding  genera- 
tions and  to  discover,  if  possible,  just  what  are  the 
carriers  of  the  heritage  from  one  generation  to  another. 
To  this  end  it  will  be  necessary  in  the  first  place 
to  take  up  what  is  meant  by  the  "cell  theory." 

2.   The  Cell  Theory 

In  1838-1839  the  "cell  theory"  of  Schleiden  and 
Schwann,  which  aflBrms  that  all  organisms,  both 
plant  and  animal,  are  made  up  of  cellular  units, 
had  its  birth. 

14 


THE   CARRIERS   OF   THE   HERITAGE        15 


Robert  Hooke,  as  early  as  1665,  had  described 
*' little  boxes  or  cells  distinguished  from  one  another" 
which  he  saw  in  thin  slices  of  cork,  and  to  him  is 
due  the  rather  unfortunate  use  of  the  term  "cell" 
which  has  survived  in  biological  writings  to  this  day. 
The  reason  this  term  is  unfortunate  is  because  walls, 
which  are  ordinarily  the  characteristic  feature  of  any 
cell,  such  as  a  prison  cell,  are  usually  the  least  im- 
portant part  of  the  structure  of  a  living  cell,  often 
indeed  being  entirely  absent. 

3.   A  Typical  Cell 

A  typical  undifferentiated  cell  is  represented 
diagrammatically  in  Figure  3.  Near  the  center  of 
the    cell   the    nucleus    is    shown     surrounded    by   a 

Cefl    wall 

C«^to  plasm 
Centrosome 
Nuclear  membrane 

--Nucleus 
.-Chromatin  network 


Fig.  3.  —  Diagram  of  a  typical  cell. 

nuclear  membrane.  The  nucleus,  in  common  with 
the  enveloping  cytoplasm,  is  made  up  of  living 
substance  called  protoplasm  (Hugo  von  Mohl,  1846), 
and  around  the  whole    there  is    usually  formed  a 


16  GENETICS 

wall  or  membrane  which  serves  to  separate  one  cell 
from  another.  Within  the  protoplasm  there  may 
be  a  considerable  amount  of  non-living  substance 
in  the  form  of  salts,  pigments,  oil-drops,  water,  and 
other  inclusions  of  various  kinds. 

The  nucleus  is  to  be  regarded  as  the  headquarters 
of  the  whole  cell,  since  changes  which  the  cell  under- 
goes seem  to  be  initiated  in  it,  while  cells  deprived  of 
their  nuclei  cannot  long  survive.  A  single  instance 
will  serve  to  show  the  vital  part  which  the  nucleus 
plays  in  the  life-history  of  the  cell.  In  1883,  Gruber 
found  that  after  rocking  a  thin  cover-glass  back  and 
forth  in  a  drop  of  water  containing  a  collection  of  the 
protozoan  Stentor,  which  has  a  long  chain-like  nucleus, 
these  tiny  animals  could  thus  be  cut  into  fragments, 
which  would  in  some  instances  recover  from  the 
operation  and  regenerate  into  complete  individuals. 
Only  those  pieces,  however,  which  contained  a  frag- 
ment of  the  nucleus  regenerated  into  new  Stentors, 
while  pieces  of  relatively  large  size  which  lacked  a  frag- 
ment of  nuclear  substance  verv  soon  disintegrated. 

The  nucleus,  it  should  be  said,  is  made  up  of  more 
than  one  substance,  a  fact  that  is  easily  demonstrated 
by  processes  of  staining,  in  which  certain  dyes, 
through  chemical  union,  stain  a  part  but  not  the 
whole  of  the  nuclear  substance.  The  part  most 
easily  stained  is  called  chromatin,  that  is  "colored 
material,"  and  during  certain  phases  of  cell  life  the 
chromatin  masses  together  within  the  nucleus  into 
visibly  definite  structures  or  bodies  termed  chromo- 
somes. 


THE   CARRIERS   OF  THE   HERITAGE        17 

Throughout  all  the  various  cells  that  make  up  the 
individuals  of  any  one  species  these  chromosomes 
appear  to  be  practically  constant  in  number  with  some 
exceptions  to  be  mentioned  later  in  connection  with 
sex.  This  law  of  the  constant  chromosome  number 
for  any  species  was  first  stated  by  Boveri  in  1900. 

The  chromosomes  of  different  organisms  vary  in 
number  from  two  in  the  worm  Ascaris  up  to  perhaps 
1600,  according  to  Haecker  ('09),  in  certain  radiolaria. 
Species  which  apparently  are  closely  related  may 
differ  widely  with  respect  to  the  number  of  their 
chromosomes,  while  species  of  unquestionably  re- 
mote relationship  may  have  an  identical  number  of 
chromosomes  in  each  of  their  cells.  The  number  of 
chromosomes  characteristic  for  a  species,  therefore, 
is  in  no  way  an  index  to  the  complexity  or  degree  of 
differentiation  of  the  species. 

Besides  the  nucleus  there  may  often  be  identified 
in  the  cytoplasm  of  the  animal  cell  a  tiny  body  known 
as  the  centrosome.  At  certain  times  in  the  life-cycle 
of  a  cell  the  centrosome  becomes  the  focal  point  of 
peculiar  radiating  lines,  which  play  an  important 
part  in  the  behavior  of  the  cell,  particularly  during 
the  period  of  division. 

Every  cell  passes  through  a  cycle  of  life  which  may 
be  compared  with  that  common  to  individuals.  It 
is  born  from  another  cell ;  passes  through  a  vigorous 
youth  characterized  by  growth  and  transformation ; 
attains  maturity  when  the  metamorphoses  of  its 
earlier  life  give  place  to  a  considerable  degree  of 
stability ;   and  finally,  after  a  more  or  less  extended 


18  GENETICS 

period  of  normal  activity  old  age  ensues,  and  death 
completes  the  cycle.  In  most  instances,  however, 
before  this  final  phase  is  reached,  the  cell  gives  place 
to  daughter-cells  through  fission,  after  the  manner 
of  most  protozoans,  and  a  new  cell  cycle  is  begun. 

Sometimes  the  road  of  differentiation  has  been 
traveled  so  far  that  it  is  apparently  impossible,  as 
in  the  case  of  the  complicated  brain-cells,  to  retrace 
these  steps  of  differentiation  and  begin  again.  In 
such  instances  the  outfit  of  cells  provided  in  the  em- 
bryo determines  the  numerical  limit  of  the  cells 
available  throughout  life.  When  this  supply  is  ex- 
hausted no  more  cells  appear  to  replace  those  w^hich 
have  been  worn  out. 

4.   Mitosis 

The  ordinary  process  by  which  two  cells  are  made 
out  of  one  is  termed  mitosis.  It  occurs  constantly, 
and  particularly  during  growth,  in  all  cellular  organ- 
isms. A  series  of  diagrams,  modified  from  Boveri, 
illustrating  the  typical  phases  of  mitosis  is  given 
in  Figures  4  to  13. 

The  resting  cell  (Fig.  4)  is  characterized  by  the 
presence  of  a  nuclear  membrane,  a  single  centrosome, 
and  by  a  chromatin  network  within  the  nucleus.  In 
the  beginning  of  the  prophase  (Fig.  5)  the  centrosome 
has  divided  into  two  parts,  while  in  the  early  prophase 
(Fig.  6)  the  two  centrosomes  have  moved  farther 
apart  and  definite  separate  chromosomes  have  formed 
out  of  the  chromatin  network.  The  prophase  proper 
(Fig.  7)  is  marked  by  the  vanishing  of  the  nuclear 


THE   CARRIERS   OF   THE   HERITAGE        19 


membrane  and  the  more  compact  form  of  the  chromo- 
somes. At  the  end  of  the  prophase  (Fig.  8)  the  chro- 
mosomes have  come  to  lie  at  the  equator  of  the  cell. 


F»g. 4.  TFie  resting  cell  Rg.  5.  Be^innin^  Prophase    Fl J.6.  Earlij  Prophase 


n^.7  Prophase 


ng.8.  End  of  Prophase  nj.9.  Metaphase 


Pig.  10.  Beginning  Anaphase 


Rg.ll.  Anaphase 


R^.12   Beginning  Telophase  n^.i3.  End  of  Telophase 

Figs.  4-13. — Diagrams  illustrating  mitosis.    After  Boveri. 

being  connected  by  the  mantle  fibers  with  the  cen- 
trosomes,  each  of  which  has  now  come  to  occupy  a 
polar  position.  In  the  metaphase  (Fig.  9)  the  chromo- 
somes split  lengthwise,  and  at  the  beginning  of  the 


20  GENETICS 

anaphase  (Fig.  10)  these  half  chromosomes  commence 
to  separate  from  each  other  and  to  move  toward 
the  poles,  while  the  mantle  fibers  shorten.  During 
the  anaphase  (Fig.  11)  the  cell  body  lengthens  and 
begins  to  divide,  w^iile  the  migration  of  the  half 
chromosomes  tow^ard  the  poles  is  completed.  In 
the  beginning  of  the  telophase  (Fig.  12)  the  half 
chromosomes  grow  until  they  attain  full  size  and 
the  division  of  the  cell  body  into  two  parts  becomes 
complete.  The  mantle  fibers  have  disappeared  and 
the  nuclear  membrane  begins  to  re-form  around  the 
chromosomes.  Finally,  at  the  end  of  the  telophase 
(Fig.  13)  the  nuclear  membrane  becomes  complete, 
the  chromosomes  break  up  into  a  chromatin  network, 
and  two  resting  cells  take  the  place  of  the  single  one 
with  which  the  process  began  (Fig.  4). 

5.   Amitosis 

Amitosis,  or  the  formation  of  two  cells  from  one 
without  the  machinery  of  mitosis,  is  comparatively 
rare.  It  occurs  in  certain  rather  isolated  instances 
among  animals  and  plants,  particularly  in  old  cells 
late  in  their  life-cycle  or  in  cells  that  are  on  the  road 
to  degeneration.  When  amitosis  takes  the  place 
of  the  more  elaborate  process  of  mitosis  it  is  fre- 
quently, though  not  always,  a  signal  of  the  death- 
warrant  for  that  particular  cell. 

6.   Sexual  Reproduction 

The  mechanism  by  means  of  which  two  cells  unite 
to  make  one  in  sexual  reproduction  is  quite  as  com- 


THE   CARRIERS   OF  THE   HERITAGE        21 

plicated  as  tliat  of  mitosis  by  which  one  cell  is  trans- 
formed into  two. 

In  sexual  reproduction  there  are  two  kinds  of  germ- 
cells,  the  egg  and  the  spermatozoan  respectively, 
which  take  part  in  producing  a  new  organism.  These 
cells  are  structurally  unlike  each  other  in  nearly 
every  particular,  but  each  is  a  true  cell,  which  von 
Kolliker  made  clear  as  early  as  1841,  and  each  has 
typically  the  same  number  of  chromosomes  in  its 
nucleus,  a  fact  more  recently  determined  by  van 
Beneden  in  1883. 

The  egg-cell  is  often  supplied  with  one  or  more 
envelopes  of  protective  or  nutritive  function,  and  it  is 
usually  distended  with  stored  up  yolk,  in  consequence 
of  which  it  is  comparatively  large  and  stationary. 
The  result  is  that  whatever  locomotion  is  necessary 
to  bring  the  two  cells  together  for  union  devolves 
upon  the  sperm-cell.  Consequently  the  sperm-cells 
are  practically  nuclei  with  locomotor  tails  of  cyto- 
plasm, and  frequently,  in  addition,  with  some  struc- 
tural modification  for  boring  a  way  into  the  egg-cell. 
They  are,  moreover,  much  more  numerous  than  the 
egg-cells,  so  that  although  many  go  astray,  never 
fulfilling  their  mission,  the  chances  are  nevertheless 
good  that  some  one  of  them  will  reach  the  egg  and 
effect  fertilization. 

Ordinarily  only  one  sperm  enters  the  egg,  but 
when  several  succeed  in  penetrating  into  the  cyto- 
plasm only  one  proceeds  to  combine  w^ith  the  egg 
nucleus,  that  is,  only  one  sperm  nucleus  is  normally 
concerned  in  the  essential  process  of  fertilization. 


22  GENETICS       ^ 

It  was  formerly  thought  by  the  school  of  "ovists" 
that  in  fertilization  the  essential  process  is  a  stimu- 
lation of  the  all  important  egg  by  the  sperm.  The 
opposing  school  of  "spermists,"  on  the  other  hand, 
regarded  the  egg  simply  as  a  nutritive  cell  the  func- 
tion of  which  is  to  harbor  the  all  important  sperm. 
It  is  now  known  that  both  the  egg-  and  the  sperm-cell 
are  equally  concerned  in  fertilization,  which  consists 
in  the  union  of  their  respective  nuclei  within  the 
cytoplasm  of  the  egg. 

7.    Maturation 

Certain  preliminary  changes  of  a  preparatory 
nature,  termed  maturation,  regularly  precede  the 
union  of  the  nuclei  of  the  two  sex-cells  in  fertiliza- 
tion. 

These  maturing  changes  result  in  reducing  the 
outfit  of  chromosomes  in  each  sex-cell  to  one  half  the 
original  number,  a  proces  which  is  necessary  in 
order  to  maintain  the  chromosome  count  which  is 
characteristic  for  any  particular  species  and  which  is 
known  to  exist  unbroken  from  generation  to  genera- 
tion. If  there  were  no  such  reduction,  then  the 
fertilized  egg,  formed  by  the  union  of  egg  and  sperm 
nuclei,  would  contain  double  the  characteristic 
number  of  chromosomes,  and  during  the  formation  of 
a  new  individual,  the  number  in  all  the  cells  arising 
by  mitosis  from  such  a  fertilized  egg  would  like- 
wise be  double.  When  the  germ-cells  of  such  indi- 
viduals unite  in  fertilization,  the  original  number  of 
chromosomes  would   be   quadrupled,  and   so   on   in 


THE  CARRIERS   OF   THE   HERITAGE        23 


geometric  progression  throughout  subsequent  genera- 
tions. In  1883,  too  late  for  Darwin  to  learn  of  it, 
van  Beneden  discovered  the  important  fact  that  the 


Primordial  Sex  Ceils 


V 


Manvj  similar   cell  divisions 

Maturation^ 
— — = — =\^ 

Sperinatocyte      1-  OocijU 
Spermatids    2.-Oocijte 


V 


'^bortiva  E^* 


Fig.  14.  —  Scheme  to  illustrate  maturation  of  germ-cells. 

mature  germ-cells,  as  expected,  actually  contain  only 
half  the  normal  number  of  chromosomes. 

The  mature  egg-  or  sperm-cell,  with  half  its  normal 
number  of  chromosomes,  is  termed  a  gamete  (marry- 


24  GENETICS 

ing  cell),  while  the  fertilized  egg  which  is  formed  by 
the  union  of  two  gametes  (mature  egg-  and  sperm- 
cell),  and  which  consequently  has  the  characteristic 
number  of  chromosomes,  is  called  a  zygote  (yoked 
cell). 

A  diagrammatic  representation  of  the  process  of 
maturation  is  shown  in  Figure  14. 

The  number  of  chromosomes  (not  shown  in  the 
diagram)  remains  constant  in  each  germ-cell  respec- 
tively until  the  division  of  spermatids  into  sperma- 
tozoa, and  of  the  second  oocytes  into  mature  eggs  and 
second  polar  cells,  when  it  is  reduced  to  one  half  the 
normal  number.  As  spermatozoan  and  mature  egg 
unite  in  fertilization,  the  original  number  of  chromo- 
somes is  restored  in  the  fertilized  egg  (zygote) . 

8.    Fertilization 

The  stages  concerned  in  a  typical  case  of  fertiliza- 
tion, according  to  Boveri,  are  illustrated  in  Figures 
15  to  23. 

In  Figure  15  the  "head"  and  the  "middle  piece" 
of  the  sperm-cell  have  penetrated  into  the  egg  cyto- 
plasm, while  in  Figure  16  the  tail  of  the  sperm-cell 
has  become  lost  and  the  middle  piece,  which  furnished 
the  centrosome,  has  rotated  180°  so  that  it  lies 
between  the  nucleus,  or  head,  of  the  sperm-cell  and 
that  of  the  egg-cell.  Figure  17  shows  an  increase  in 
the  size  of  the  sperm  nucleus  and  a  division  of  the 
centrosome  into  two  parts  which  begin  to  migrate 
towards  the  poles.  This  process  of  polar  migration 
of  the  centrosomes  is  carried  further  in  Figure  18  as 

f  JWPEMT  UBRARt 


THE   CARRIERS   OF   THE   HERITAGE        25 


fiig.18.  Approach  of  Sperm 
Nucleus 


Pig.lS.  Entn^  of  Sperm  Ff^lG.  Loss  of  Sperm  Tall       R^.IZ  Division  of  Cenlrosome 


f?g.  19,  Increase  of  Sperm         Fi^.ZO.  Formation  of 
Nucleus  Chromosomes 


Fi*g.2.I.  Splitting  of  Chromosomes  H^.  £2.  Anaphase 


n^.SS.Two-celled  Sla^e 
Figs.  15-23.  —  Diagrams  illustrating  fertilization.    After  Boveri. 


26  GENETICS 

well  as  the  increase  in  the  size  of  the  sperm  nucleus, 
until  in  Figure  19  the  process  is  complete  so  that  the 
centrosomes  have  assumed  a  polar  position  and  the 
sperm  nucleus  is  equal  in  size  to  the  egg  nucleus 
and  lies  in  contact  with  it.  In  Figure  20  the  chro- 
matin network  of  the  two  nuclei  has  formed  into  an 
equal  number  of  chromosomes  which  in  each  case 
is  half  the  number  characteristic  for  the  species. 
Figure  21  shows  the  complete  disappearance  of  the 
nuclear  membrane,  a  process  that  had  already  begun 
in  the  preceding  figure,  and  also  the  arrangement  of 
the  chromosomes,  connected  with  mantle  fibers,  in  the 
equatorial  plane  where  the  former  split  longitudinally. 
In  Figure  22,  when  the  half  chromosomes  thus  formed 
pull  apart  and  migrate  toward  the  poles,  the  segmenta- 
tion of  the  fertilized  egg  has  begun,  and  there  finally 
occurs,  as  shown  in  Figure  23,  the  two-celled  stage 
following  fertilization  in  which  each  cell  contains  the 
normal  number  of  chromosomes,  half  of  which  came 
from  the  egg  and  half  from  the  sperm. 

9.   Parthenogenesis 

Fertilization  is  by  no  means  an  essential  process  in 
the  formation  of  a  new  individual,  even  in  those  ani- 
mals which  produce  both  eggs  and  sperms.  Many 
animals  and  plants  reproduce  parthenogenetically, 
that  is,  the  egg-cell  may  develop  without  first  uniting 
with  a  sperm-cell.  In  these  instances  the  chromo- 
somes of  the  egg  are  not  halved  during  maturation, 
and  the  offspring,  therefore,  have  the  same  number 


THE   CARRIERS   OF   THE   HERITAGE        27 

of  chromosomes  as  the  parent,  since  they  are  simply 
fragments  of  the  parent. 

Professor  Loeb,  by  the  use  of  certain  chemicals, 
has  succeeded  in  doing  artificially  what  apparently 
is  never  accomplished  in  nature,  namely,  making  an 
egg  that  normally  requires  fertilization  develop  par- 
thenogenetically. 

10.   The  Hereditary  Bridge 

Whatever  may  ultimately  prove  to  be  deter- 
miners of  the  hereditary  characters  which  appear  in 
successive  generations,  it  is  obvious  that,  in  any 
event,  such  determiners  must  be  located  in  the  zygote, 
that  is,  in  the  fertilized  egg.  This  single  cell  is  the 
actual  bridge  of  continuity  between  any  parental 
and  filial  generation.  Moreover,  it  is  the  07ily 
bridge. 

In  the  majority  of  animals  the  egg  develops  en- 
tirely outside  of  and  independent  of  the  mother, 
thus  limiting  to  the  egg-cell  itself  all  possible  mater- 
nal contributions  to  the  offspring.  Although  there 
is  abundant  evidence  that  half  of  the  filial  char- 
acteristics come  from  the  male  parent,  the  only 
actual  fragment  of  the  paternal  organism  given  over 
to  the  new  individual  is  the  single  sperm-cell,  which 
unites  with  the  egg  in  fertilization,  and  the  whole 
of  this  even  is  not  usually  concerned  in  the  process 
of  fertilization.  The  entire  factor  of  heritage  is 
packed  into  the  two  germ-cells  derived  from  the  re- 
spective parents  and,  in  all  probability,  into  the 
nuclei  of  these  germ-cells,  since  the  nuclei  are  ap- 


28  GENETICS 

parently  the  only  portions  of  these  cells  that  in- 
variably take  part  in  fertilization.  To  the  new 
individual  developing  by  mitosis  from  the  fertilized 
egg  into  an  independent  organism,  the  factors  of 
environment  and  training  referred  to  in  Figure  1 
are  subsequently  added. 

When  it  is  remembered  that  the  human  egg- cell 
is  only  about  2Vth  of  an  inch  in  diameter,  a  gigantic 
size  as  compared  with  that  of  the  human  sperm-cell, 
and,  furthermore,  when  one  passes  in  rapid  review 
the  marvelous  array  of  characteristics  which  make 
up  the  sum  total  of  what  is  obviously  inherited  in 
man,  the  wonder  grows  that  so  small  a  bridge  can 
stand  such  an  enormous  traffic.  A  sharp-eyed  patrol 
of  this  bridge  as  the  strategic  focus  of  heredity  is 
proving  to  be  one  of  the  most  effective  points  of 
attack  in  the  entire  campaign  of  genetics. 

It  is  not  desirable  at  this  time  to  discuss  possible 
ways  in  which  the  determiners  of  the  heritage,  what- 
ever they  may  be,  are  originally  packed  into  the 
germ-cells,  for  this  question  can  be  more  conven- 
iently considered  in  a  later  connection.  It  is  im- 
portant at  present,  however,  to  emphasize  the  ob- 
vious conclusion  that  determiners  of  heredity  must 
inevitably  be  present  in  the  germ-cells  in  order  to 
account  for  the  fact  of  ''organic  resemblance  based 
on  descent"  between  parents  and  their  progeny. 

11.   The  Determiners  of  Heredity 

What  are  the  determiners  of  hereditary  qualities  ? 
Do  they  actually  exist  in  the  germ-cells  as  visible 


THE   CARRIERS   OF   THE   HERITAGE        29 

entities,  and  is  there  such  a  thing  as  a  mechanical 
basis  for  heredity  as  the  German  embryologist 
Wilhelm  His  suggested  years  ago  when  he  wrote : 
"It  is  a  piece  of  unscientific  mysticism  to  suppose 
that  heredity  will  build  up  an  organism  without 
mechanical  means"  ?  Can  we  find  these  determiners 
by  the  aid  of  microscopes  and  differential  stains, 
or  are  they  some  sort  of  intangible  entities,  such  as 
enzymes  or  hormones  or  the  like,  which  only  the 
chemist  can  detect  ? 

Whatever  the  answer  to  these  questions,  it  may  at 
least  be  affirmed  that  the  determiner  represents  the 
adult  structure  without  resembling  it.  It  is  something 
which  controls  the  unfolding  of  the  developing  or- 
ganism with  respect  to  both  quantity  and  quality, 
and  which  also  governs  the  time  and  rate  of  appear- 
ance of  its  various  characteristics  so  that  certain 
combinations  rather  than  others  shall  come  about 
in  definite  sequence.  To  use  the  words  of  Conklin : 
"The  mechanism  of  heredity  is  the  mechanism  of 
differentiation." 

12.   The  Chromosome  Theory 

Certain  investigators,  who  seek  a  morphological 
basis  for  heredity,  regard  the  chromosomes  as  the  car- 
riers of  the  heritage;  in  other  words,  as  the  source 
of  the  determiners  of  ontogeny  or  the  effective 
factors  in  the  process  of  differentiation. 

A  few  of  the  grounds  for  this  theory  are  briefly 
indicated  below. 

First:    In  spite  of  the  great  relative  difference  in 


30  GENETICS 

size  between  the  egg-cell  and  the  sperm-cell,  in  hered- 
ity the  two  are  practically  equivalent,  as  has  been 
repeatedly  shown  by  making  reciprocal  crosses  be- 
tween the  two  sexes.  The  only  features  that  are 
apparently  alike  in  both  the  germ-cells  are  the 
chromosomes.  The  inference  is,  therefore,  that  they 
contain  the  determiners  which  are  the  causal  factors 
for  the  equivalence  of  adult  characters  in  heredity. 
The  existence  of  an  extra  chromosome  in  probable 
connection  with  the  matter  of  sex  is,  as  will  be  pointed 
out  later,  an  exception  to  the  exact  chromosome 
equivalence  of  the  two  sexes,  which  only  goes  to 
strengthen  the  supposition  that  the  chromosomes  are 
the  carriers  of  hereditary  qualities  since  extra  chromo- 
somes are  always  associated  with  the  character  of  sex. 

Second:  The  process  of  maturation,  which  always 
results  in  halving  the  chromosome  material  of  the 
germ-cells  as  a  preliminary  step  to  fertilization,  is 
a  series  of  complicated  manoeuvers  not  practised  by 
other  cells.  During  this  process  no  other  part  of 
the  cells  appears  to  play  so  consistent  and  important 
a  role  as  the  chromosomes.  Provided  they  act  as 
hereditary  carriers,  their  peculiar  behavior  during 
maturation  is  just  what  is  needed  to  bring  together 
an  entire  complement  of  hereditary  determiners  out 
of  partial  contributions  from  tw^o  parental  sources. 

Third :  Sometimes  abnormal  fertilization  occurs,  as 
in  the  case  when  two  or  more  sperm-cells,  instead  of 
one,  enter  the  egg  cytoplasm  and  unite  with  the  egg 
nucleus.  This  unusual  performance  has  been  artifi- 
cially induced  by  chemical  means  in  the  case  of  sea- 


THE   CARRIERS   OF   THE   HERITAGE        31 

urchins'  eggs.  The  fertilized  egg,  or  zygote,  thus 
formed  with  an  excess  of  male  chromosomes,  re- 
sults in  the  development  of  abnormal  larvae.  It  is 
thought  that  a  causal  connection  may  exist,  there- 
fore, between  the  additional  male  chromosomes  in 
the  fertilized  ovum  and  the  abnormalities  of  the 
progeny. 

Fourth:  The  fact  that  chromosomes  may  retain 
their  individuality  throughout  the  complicated  phases 
of  mitosis,  as  has  been  proven  in  some  instances, 
agrees  with  the  corresponding  fact  that  certain 
characteristics  of  the  somatoplasm  maintain  their 
individuality  from  generation  to  generation. 

Moreover,  certain  chromosomes  in  the  fertilized 
egg  have  been  identified  with  particular  features  in 
the  adult  developing  from  that  egg.  Tennent  sum- 
marizes his  recent  work  on  Echinoderms  (1912) 
by  the  statement  that  from  a  knowledge  of  the 
chromosomes  in  the  parental  germ-cells,  particular 
characters  in  the  adult  hybrids  may  be  predicted, 
and,  conversely,  that  from  the  appearance  of  sexually 
mature  hybrids  the  character  of  certain  chromosomes 
in  their  germ-cells  may  be  predicted. 

Again,  the  correlation  of  a  particular  chromosome 
in  the  germ-cells  with  a  definite  adult  character, 
namely  sex,  has  been  repeatedly  demonstrated  in 
connection  with  the  so-called  "extra  chromosome" 
to  which  reference  has  already  been  made. 

Fifth:  Finally,  excellent  evidence  of  a  definite 
causal  connection  between  certain  chromosomes  of 
the  germ-cells  and  particular  somatic  characters  has 


32  GENETICS 

been  furnished  by  certain  critical  experiments  upon 
the  eggs  of  sea-urchins.  Boveri  found  that  he  was 
able  in  some  instances  to  shake  out  the  nuclei  bodily, 
chromosomes  and  all,  from  the  mature  eggs  of  the 
sea-urchin,  Splicer  echinus,  and  when  there  was  added 
in  sea  water  to  such  enucleated  eggs  the  sperm-cells 
of  an  entirely  different  genus  of  sea-urchin,  namely. 
Echinus,  the  Echinus  sperm-cells  entered  the  Sphcer- 
echinus  eggs,  which  had  been  robbed  of  their  nuclei, 
and  from  this  peculiar  combination  larvae  developed 
which  exliibited  only  Echinus  characters! 

Such  cuniulative  circumstantial  evidence  as  the 
foregoing  has  convinced  many  that  in  the  chromo- 
somes we  have  visibly  before  us  the  carriers  of 
heredity. 

Several  biologists,  however,  raise  an  objecting 
voice  to  this  theory,  protesting  against  the  mo- 
nopoly of  the  heritage  by  the  chromosomes.  They 
point  out  that  there  always  exists  an  intimate 
physiological  relationship  between  the  nucleus  and 
the  cytoplasm,  and  that  it  is  unreasonable  to  expect 
the  isolation  of  one  from  the  other,  since  the  two 
must  always  act  together  as  parts  of  an  organic  cell 
unit. 

In  sexual  reproduction,  moreover,  some  small 
amount  at  least  of  spermatic  cytoplasm  in  the  form 
of  the  so-called  "middle  piece,"  which  is  situated 
between  the  head  and  the  tail  of  the  sperm-cell 
(Fig.  15),  may  enter  the  egg  about  to  be  fertilized 
along  with  the  sperm  "  head  "  or  nucleus,  containing 
the  chromosomes.     In  this  way  the  cytoplasm  of  the 


THE   CARRIERS   OF   THE   HERITAGE        33 

male  sperm-cell  may  not  necessarily  be  entirely 
excluded  from  taking  part  in  the  formation  of  the 
zygote.  As  a  matter  of  fact,  this  extra-nuclear  part 
of  the  sperm-cell  sometimes  apparently  forms  the 
centrosome  of  the  fertilized  egg  and  in  consequence 
may  have  a  hand,  as  well  as  the  nucleus  with  the 
chromosomes,  in  determining  what  follows. 

13.   The  Enzyme  Theory  of  Heredity 

It  is  not  unlikely  that  the  key  to  this  whole  prob- 
lem will  be  furnished  by  the  biochemists  and  that  the 
final  analysis  of  the  matter  of  the  heritage-carriers 
will  be  seen  to  be  chemical  rather  than  morpho- 
logical in  nature. 

It  has  been  found  that  the  blood  of  greyhounds 
and  dachshunds  is  chemically  different,  although 
from  a  morphological  point  of  view  it  is  apparently 
identical.  The  idea  of  "individual  albumen"  or 
"protein  specificity"  for  each  animal  of  a  species, 
to  say  nothing  of  the  animals  of  different  species, 
has  been  advanced  as  not  improbable. 

Miescher  has  shown  that  an  albumen  compound 
having  only  forty  carbon  atoms,  a  number  by  no 
means  unusual,  would  make  possible  a  million  com- 
binations of  atoms  or  isomers. 

The  possibilities  in  this  direction  seem  to  be  un- 
limited if  we  take  into  consideration  those  invisible 
actuators  of  chemical  processes,  the  enzymes,  which 
the  chemist  brings  forw^ard  with  the  prodigality  of 
an  astronomer  dealing  in  star-dust,  to  explain  dif- 
ferent chemical  reactions. 


34  GENETICS 

Montgomery  has  suggested  that  the  chromosomes 
themselves  may  be  masses  of  enzymes  although,  ac- 
cording to  the  chemist,  enzymes  are  not  morpho- 
logical entities,  since  they  seem  to  be  able  to  flourish 
and  maintain  their  identity  while  bringing  about 
chemical  reactions  in  their  neighborhood  without 
being  visibly  demonstrable. 

As  said  before,  it  is  quite  likely  that  in  the  final 
analysis  heredity  will  be  reduced  to  a  series  of  chemi- 
cal reactions  dependent  upon  the  manner  in  which 
various  enzymes  initiate,  retard,  or  accelerate  suc- 
cessive chemical  combinations  occurring  in  the  pro- 
toplasm. When  the  same  enzymes  act  upon  the 
same  chemical  combinations  in  successive  genera- 
tions, they  bring  about  that  "organic  resemblance" 
known  as  heredity. 

E.  B.  Wilson,  whose  brilliant  work  in  the  entire 
field  of  cell  activity  makes  it  possible  for  him  to 
speak  with  authority,  has  recently  said  :  "The  es- 
sential conclusion  that  is  indicated  by  cytological 
study  of  the  nuclear  substance  is,  that  it  is  an  ag- 
gregate of  many  different  chemical  components 
which  do  not  constitute  a  mere  mechanical  mixture, 
but  a  complex  organic  system  and  which  undergo 
perfectly  ordered  processes  of  segregation  and  dis- 
tribution in  the  cycle  of  cell  life.  That  these  sub- 
stances play  some  definite  role  in  determination  is 
not  mere  assumption,  but  a  conclusion  based  upon 
direct  cytological  experiment  and  one  that  finds 
support  in  the  results  of  modern  chemical  research." 


THE   CARRIERS   OF   THE   HERITAGE        35 

14.    Conclusion 

The  supposition  that  the  chromosomes,  with  cer- 
tain chemical  reservations,  are  the  morphological 
carriers  of  the  heritage  forms  an  excellent  working 
hypothesis,  and  this  chapter  may  suitably  be  closed 
with  a  second  quotation  from  Professor  Wilson. 
"  In  my  view  studies  in  this  field  are  at  the  present 
time  most  likely  to  be  advanced  by  adopting  the 
comparatively  simple  hypothesis  that  the  nuclear 
substances  are  actual  factors  of  reaction  by  virtue 
of  their  specific  chemical  properties ;  and  I  think 
that  it  has  already  helped  us  to  gain  a  clearer  view 
of  some  of  the  most  puzzling  problems  of  genetics." 


CHAPTER  III 

VARIATION 

1.   The  Most  Invariable  Thing  in  Nature 

In  the  introductory  chapter  it  was  shown  that 
"organic  resemblance  based  on  descent,"  by  which 
is  meant  heredity,  is  due  principally  to  the  fact  that 
offspring  are  material  continuations  of  their  parents 
and  consequently  may  be  expected  to  be  like  them. 
The  fact  that  this  is  the  case  in  the  great  majority  of 
instances  has  given  rise  to  the  popular  formula, 
"like  produces  like,"  as  a  rule  of  heredity. 

But  this  formula  by  no  means  always  fits  the  facts. 
Like  often  produces  something  apparently  unlike. 
For  instance,  two  brown-eyed  parents  may  produce 
a  blue-eyed  child,  although  brown-eyed  children  are 
more  usual  from  such  a  parentage.  It  is  a  common 
experience,  indeed,  for  breeders  of  plants  and  animals  ^ 
to  meet  with  continual  difficulties  in  getting  or- 
ganisms to  "breed  true." 

On  the  other  hand,  it  is  exactly  these  variations 
which  so  constantly  interfere  with  breeding  true 
that  furnish  the  sole  foothold  for  improvement.  If 
all  organisms  did  breed  strictly  true,  one  generation 
could  not  stand  on  the  shoulders  of  the  preceding 
generation,  and  there  would  be  no  evolutionary 
advance. 

36 


VARIATION  37 

The  most  invariable  thing  in  nature  is  variation. 
This  fact  is  at  once  the  hope  and  the  despair  of  the 
breeder  who  seeks  to  hold  fast  to  whatever  he  has 
found  that  is  good  and  at  the  same  time  tries  to 
find  something  better.  When  the  similarities  and 
dissimilarities  between  succeeding  generations  are 
clear,  then  heredity  can  be  explained.  The  entire 
subject  of  variation  is  intimately  and  inevitably 
bound  up  with  any  consideration  of  genetics. 

2.   The  Universality  of  Variation 

Much  of  the  variation  in  nature  is  patent  to  the 
most  casual  observer,  but  it  requires  a  trained  eye  to 
see  the  universal  extent  of  many  minor  differences. 
A  flock  of  sheep  may  all  look  alike  to  a  passing  stran- 
ger, but  not  to  the  man  who  tends  them.  A  dozen 
blue  violet  plants  from  different  localities  might 
easily  be  identified  by  the  amateur  botanist  as  be- 
longing to  the  same  species  when,  to  a  specialist 
on  the  genus  Viola,  unmistakable  differences  would 
doubtless  be  clearly  apparent. 

The  fact  that  every  attempt  at  an  intimate  ac- 
quaintance with  any  group  of  organisms  whatsoever 
invariably  reveals  previously  unrecognized  varia- 
tions, indicates  that  variability  is  much  more  wide- 
spread in  nature  than  is  commonly  believed. 
,  The  key  to  Japanese  art,  as  pointed  out  by  Dr. 
Nitobe,  consists  in  being  natural  and  in  faithfully 
copying  nature.  It  is  for  this  reason  that  the  Jap- 
anese artist   makes   each   object   that  he  produces 


38  GENETICS 

unique,  because  nature  herself,  whom  he  strives  to 
follow,  never  duplicates  anything. 

The  Bertillon  system  of  personal  identification  is 
based  upon  the  constancy  of  minor  variations  found 
in  each  individual.  Its  importance  is  shown  in 
Figure  24.  The  faces  of  the  criminals  there  pictured 
would  be  easily  confused  by  the  ordinary  observer,  but 
an  examination  of  their  thumb  prints  shows  unmis- 
takable differences  between  these  three  individuals. 

3.   Kinds  of  Variation 

A  brief  enumeration  of  some  of  the  kinds  of  varia- 
tion will  reveal  their  diverse  character. 

a.  With  respect  to  their  nature  variations  may  be 
morphological,  physiological,  or  psychological.  Under 
morphological  variations  are  included  differences  in 
shape,  size,  or  pattern  as  well  as  differences  in  number 
and  relation  of  constituent  parts. 

Differences  in  activity  are  of  a  physiological  nature. 
Many  animals  in  captivity  are  less  fertile  than  when 
free,  while  different  individuals  are  well  known  to 
vary  widely  with  respect  to  their  susceptibility  to 
disease.  Nageli,  for  example,  reports  the  presence  of 
tubercles  in  97  per  cent  of  the  cases  in  1^ve  hundred 
autopsies,  although  a  majority  of  the  deaths  in  ques- 
tion was  not  due  to  tuberculosis  at  all,  —  a  fact  which 
indicates  a  great  diversity  in  the  resistance  of  differ- 
ent individuals  to  the  tubercle  bacillus. 

Psychological  variations  in  man,  such  as  those 
which  determine  the  disposition  or  mental  traits  of 
individuals,  are  apparent  to  every  one. 


VARIATION  39 

b.  With  respect  to  their  duplication  variations  may 
be  single  or  multiple.  A  legless  lamb  ^  is  an  ex- 
ample of  a  single  variation  or  *' sport."  Four-leaved 
clovers,  on  the  contrary,  are  multiple  for  the  reason 
that  this  variation,  although  not  common,  neverthe- 
less occurs  frequently. 

c.  With  respect  to  their  utility  variations  may  be 
useful,  indifferent,  or  harmful  to  the  organism  possess- 
ing them.  Useful  variations  are  of  the  kind  empha- 
sized by  Darwin  as  being  effectively  made  use  of  in 
natural  selection.  Indifferent  variations,  on  the 
other  hand,  are  those  which  apparently  do  not  play 
an  important  part  in  the  welfare  of  their  possessor, 
as,  for  example,  the  color  of  the  eyes  or  of  the  hair. 
Finally,  the  degree  of  degeneration  in  certain  organs 
may  be  cited  as  an  illustration  of  harmful  variations. 
The  amount  of  closure  of  the  opening  from  the  in- 
testine into  the  vermiform  appendix  in  man  is  an  ex- 
ample of  a  harmful  variation,  since  the  larger  the 
opening,  the  greater  is  the  liability  to  appendicitis. 

d.  With  respect  to  their  direction  in  evolution  varia- 
tions may  be  either  definite  (orthogenetic)  or  indefinite 
(fortuitous) . 

Paleontology  furnishes  numerous  instances  of  the 
former  category,  such  as  the  series  of  variations 
from  a  pentadactyl  ancestor,  all  apparently  tending 
in  one  direction,  which  have  culminated  in  the  one- 
toed  horse.  The  fact  that  the  paleontologist  deals 
historically  with  a  completed  phylogenetic  series  in 
which  the  side  lines  lack  prominence,  while  the  suc- 

1  "A  Peculiar  Legless  Lamb."     Stockard.     Biol.  Bull,  xiii,  p.  288. 


40  GENETICS 

cessful  line  stands  out  with  distinctness,  makes  it  easy 
for  him  to  view  successive  variations  as  orthogenetic, 
that  is,  as  definitely  directed  in  one  course  either 
through  intrinsic  (Nageli)  or  extrinsic  (Eimer)  causes. 

Fortuitous  or  chance  variations  in  all  possible 
directions  furnish  the  repertory  of  opportunity, 
according  to  Darwin,  from  which  natural  selection 
picks  out  those  best  adapted  to  survive  in  the  strug- 
gle for  existence. 

e.  With  respect  to  their  source^  variations  may  be 
somatic  or  germinal.  Somatic,  or  body  variations, 
arise  as  modifications  due  to  environmental  factors. 
They  are  individual  differences  which  may  be  quite 
transitory  in  nature,  while  germinal  variations  may 
arise  without  regard  to  the  environment,  are  deep- 
seated,  and  of  racial  rather  than  of  individual  sig- 
nificance. 

/.  With  respect  to  their  normality  variations  may 
fall  within  expected  extremes  and  thus  be  considered 
normal,  or  they  may  be  outside  of  reasonable  expec- 
tations and  consequently  be  reckoned  as  abnormal, 
as  in  the  case  of  a  two-headed  calf. 

g.  With  respect  to  the  degree  of  their  continuity  varia- 
tions may  form  a  continuous  series,  grading  into  each 
other  by  intermediate  steps,  or  they  may  be  discon- 
tinuous in  character.  An  example  of  continuous 
variation  is  the  height  of  any  hundred  men  one  might 
chance  to  meet,  which  w ould  probably  represent  all 
intermediate  grades  from  the  highest  among  the 
hundred  to  the  lowest. 

The  number  of   segments  in  the  abdomen  of   a 


VARIATION  41 

shrimp,  on  the  other  hand,  which  may,  for  instance,  be 
either  eight  or  nine  but  cannot  be  halfway  between, 
illustrates  what  is  meant  by  discontinuous  variation. 
The  widespread  occurrence  of  this  later  category  of 
variations  has  been  pointed  out  by  Bateson  in  his 
encyclopedic  volume  "On  Materials  for  the  Study 
of  Variation." 

h.  With  respect  to  their  character  variations  may  be 
quantitative  or  qualitative.  A  six-rayed  starfish 
represents  a  quantitative  variation  from  the  normal 
number  of  five  rays,  whereas  a  red  variety  of  a  flower 
may  differ  chemically  from  a  blue  variety,  or  a  bitter 
fruit  may  differ  from  a  sweet  fruit  in  a  qualitative 
way  dependent  upon  the  chemical  constitution  of 
the  fruit  in  question. 

i.  With  respect  to  their  relation  to  an  average  stand- 
ard variations  may  have  a  fluctuating  distribution 
around  an  arithmetical  mean,  as  when  some  of  the 
offspring  have  more  and  some  less  of  the  parental 
character,  or  the  variations  in  the  progeny  may  all 
center  about  a  new  average  quite  distinct  from  the 
parental  standard  and  consequently  come  under  the 
head  of  mutations, 

j.  Finally,  and  most  important  in  the  present 
connection,  with  respect  to  heritability,  variations  may 
possess  the  power  to  reappear  in  subsequent  genera- 
tions, or  they  may  lack  that  power.  It  is  this  aspect 
of  variability  which  bears  most  directly  upon  genetics. 

Other  possible  categories  might  be  mentioned,  but 
a  suflicient  number  have  been  cited  to  show  the 
great  diversity  of  variations  in  general. 


42  GENETICS 

4.    Methods  of  Studying  Variations 

Roughly  stated,  there  are  three  ways  of  studying 
variations :  first,  Darwin's  method  of  observation 
and  the  description  of  more  or  less  isolated  cases ; 
second,  Galton's  biometric  method  of  statistical 
inquir}^ ;  and  third,  Mendel's  experimental  method. 
The  second  of  these  methods  will  be  considered  in 
this  chapter. 

5.   Biometry 

The  new  science  of  biometry,  that  is,  the  applica- 
tion of  statistical  methods  to  biological  facts,  has 
been  developed  within  recent  years.  Sir  Francis 
Galton,  Darwin's  distinguished  cousin,  may  be  re- 
garded as  the  pioneer  in  this  field  of  research,  while 
Karl  Pearson  and  his  disciples  constitute  the  modern 
school  of  biometricians. 

Although  mathematical  analysis  of  biological 
data  when  not  sufficiently  ballasted  by  biological 
analysis  of  the  same  facts  may  sometimes  lead  the 
investigator  astray,  yet  often  the  only  way  to  for- 
mulate certain  truths  or  to  analyze  data  of  some 
kinds  is  by  resort  to  statistical  methods.  Biome- 
tricians are  quite  right  in  insisting  that  it  is  frequently 
necessary  to  go  further  than  the  fact  of  variation, 
which  may  be  apparent  from  the  inspection  of  an 
individual  case,  and  to  deal  with  cumulative  evidence 
as  presented  through  statistical  analysis. 

In  matters  of  heredity,  however,  facts  as  they 
occur  in  single  cases  and  definite  pedigrees  seem  to 


VARIATION 


43 


offer  a  more  hopeful  line  of  approach  than  statistical 
generalizations.  It  is  better  to  become  acquainted 
with  the  real  parent  than  to  evolve  a  hypothetical 
"mid-parent"  mathematically.  In  this  connection 
it  is  well  always  to  bear  in  mind  the  warning  of 
Johanssen,  himself  a  past  master  in  biometry,  when 
he  writes  :  '^  Mit  Mathematik  niclit  als  Mathematik 
treiben  wir  unsere  Studien." 

6.   Fluctuating  Variation 

With  respect  to  any  measurable  character  there 
are  bound  to  be  deviations  from  an  average  con- 
dition. According  to  the  mathematical  laws  of 
chance  these  deviations  sometimes  are  plus  and 
sometimes  minus,  and  consequently  they  may  be 
termed  fluctuating  variations. 

Pearson  gives  as  a  simple  illustration  of  fluctuating 
variation  the  number  of  ribs  present  in  two  sets  of 
beech-leaves,  as  shown  below.  These  sets  were  taken 
from  two  different  trees,  and  each  contains  twenty- 
six  leaves. 


Number  of  Ribs 

13 

14 

16 

16 

17 

18 

19 

0 

Total 

First  tree     .     .     . 
Second  tree      .     . 

3 

4 

1 

9 

4 

8 

7 
2 

9 

4 

1 

26 
26 

Total  .... 

3 

4 

10 

12 

9 

9 

4 

1 

It  will  at  once  be  seen  that,  while  certain  leaves 
might  well  belong  to  either  tree,  as,  for  example,  those 
with  sixteen  ribs,  the  entire  group  of  leaves  from 


44 


GENETICS 


either  tree  is  unlike  that  of  the  other  tree.     In  the 
first  instance  the  number  of  ribs  fluctuates  around 

Number  oi 
Individuals 


35  p.       UsTor  Constants 


30 


£5 


Arithmetical  MeanCA.M)=49 
Mode  CM) -5 

Average  Deviation  (A.D.')=.5£ 
Standard  Deviation  (o-)  =  .7e4 

—      CoeKicient  oi  VariabilitY  (C.V.)=  1^1 

Formulae 


zo  - 


15 


W 


t 


A.D.  =    S(x.f) 


=T 


cr  =  rzcx^.f) 


-    c.\/  = 


n 


A.M. 

2  =  sum 

x=  deviation  of  the  class 
from  A.M. 

f  =  number  in  the  class 
n  =•  total  number 


Number  of  Rays    £  3  4  5 

Fig.  25. — The  fluctuating  variability  of  starfish  rays. 

Goldschmidt. 


6  7 

From  data  by 


eighteen  as  the  commonest  kind ;  in  the  second  case, 
around  fifteen.     Such  a  difference  could  not  easily 


VARIATION  45 

be  detected  or  expressed  by  any  other  method  than 
the  statistical  one. 

Again,  in  the  case  of  forty-seven  starfishes  all  of 
which  were  collected  from  one  locality  the  variation 
in  the  number  of  rays  proved  to  be,  according  to 
Goldschmidt,  an  amount  indicated  graphically  in 
Figure  25,  where  the  data  are  arranged  in  the  form 
of  a  so-called  frequency  polygon  or  curve. 

From  such  a  polygon  certain  constants  may  be 
computed  which  conveniently  express  in  a  single 
number,  for  purposes  of  abstract  comparison,  dis- 
tinctions that  otherwise  could  be  handled  only  in 
the  most  indefinite  way. 

Thus  in  this  instance  the  arithmetical  mean,  ex- 
pressed by  the  hypothetical  number  4.915,  a  number 
which  of  course  does  not  actually  occur  in  nature, 
is  simply  the  average  number  of  rays  in  forty-seven 
starfishes  selected  at  random. 

The  mode  which  represents  the  group  containing 
the  largest  number  of  individuals  of  a  kind,  namely, 
thirty  out  of  forty-seven,  is  five  in  this  particular 
polygon. 

The  average  deviation,  which  is  an  index  of  the 
amount  of  variation  going  on  among  the  starfishes  in 
question,  is  .52.  In  other  words,  .52  is  the  average 
amount  that  each  individual  starfish  deviates  from 
the  arithmetical  mean  of  4.915.  Although  the  one 
seven-rayed  starfish  which  happens  to  be  in  the 
lot  varies  from  the  standard  of  4.915  to  the  extent 
of  2.085  (7  —  4.915)  rays,  there  are  thirty  five-rayed 
starfishes  which  vary  only  .085  (5  —  4.915)  of  a  ray, 


46  GENETICS 

and  consequently  the  average  of  the  entire  forty- 
seven  amounts  to  .52  of  a  ray.  In  another  collec- 
tion of  starfishes  where  either  more  seven-rayed  or 
two-rayed  specimens  might  be  present,  the  average 
deviation  would  probably  be  greater. 

By  computing  the  average  deviation,  therefore, 
and  using  it  as  the  criterion  of  variation,  a  compar- 
ison of  the  variability  of  organisms  that  have  been 
taken  from  different  localities  or  subjected  to  differ- 
ent conditions  can  be  definitely  expressed. 

A  measure  of  variability  more  commonly  in  use 
by  biometricians,  since  for  mathematical  reasons  it 
is  more  accurate,  is  the  standard  deviation.  This  is 
the  square  root  of  the  sum  of  all  the  deviations 
squared,  according  to  the  formula 


in  which  x  represents  the  deviation  of  each  class  from 
the  arithmetical  mean ;  /,  the  number  of  individuals 
in  each  separate  class ;  2,  the  sum  of  the  classes ;  and 
n,  the  total  number  of  individuals.^ 

In  the  present  instance  the  standard  deviation  is 
.724,  an  arbitrary  number  that  has  valuable  sig- 
nificance only  when  brought  into  comparison  with 
standard  deviations  similarly  derived  from  other 
groups  of  starfishes. 

Such  a  variation  polygon  as  the  above  expresses 
the  law  that  the  farther  any  single  group  is  from  the 

^For  directions  explaining  the  use  of  such  formulae  see  Davenport's 
"  Statistical  Methods." 


VARIATION 


47 


mean  of  all  the  groups  making  up  the  pol^^gon,  the 
fewer  will  be  the  individuals  that  represent  it. 

7.   The  Interpretation  of  Variation  Polygons 

a.  Relative  Variability 

The  statistical  determination  of  the  relative  vari- 
ability of  two  lots  of  organisms  with  respect  to  a 
certain  character  may  be  illustrated  by  the  case  of 
the  oyster-borer  snail,  Urosalpinx  cinereus,  as  seen  in 
the  accompanying  table. 

Atlantic  and  Pacific  Shells  Compared 


Locality 


Woods 
Hole 


West  Shore 
Penzance  Point 
Nobska  Point 
Nobska  Point 
Nobska  Point 
Barnacle  Beach 
Big  Wepecket 
Mid-Wepecket 


Average  for  Mass 


Cali- 


f  Belmont  Beds 


fornia  '[  San  Francisco  Bay 


Average  for  Cal. 


Difference 


Number 
OF  Shells 


1,001 

1,002 

1,002 

1,001 

496 

998 

1,006 

500 


1,008 
520 


A.M. 


58.928 
61.718 
61.737 
61.944 
66.944 
63.932 
57.426 
57.606 


61.066 


59.051 

60.892 


59.664 


2.339 
2.737 
2.152 
2.234 
2.366 
2.604 
2.052 
2.098 


2.335 


3.023 
3.361 


3.138 


.803 


Prob- 
able 
Error 


±.0352 
±.0412 
±.0324 
±.0337 
±.0507 
±.0393 
±.0308 
±.0447 


±.0386 


±.0454 
±.0703 


±.0538 


The  obvious  conclusion  to  be  drawn  from  this 
table  is  that  the  snails  which  were  unintentionally 
carried  from  the  Atlantic  coast  to  California  in  the 


48  GENETICS 

transplantation  of  oysters  show  more  variation  in 
their  new  habitat  than  they  did  in  the  old  one  with 
respect  to  the  particular  character  measured,  namely, 
the  relative  size  of  the  mouth  aperture  compared  with 
the  height  of  the  entire  shell. ^ 

b.  Bimodal  Curves 

Sometimes  two  conspicuous  modes  make  their  ap- 
pearance in  a  frequency  polygon,  as  Jennings  found, 

f|Jonib»r 
■    of 
individuals 


25 

- 

t 

20 

- 

\ 

J5 

- 

/   \         f\ 

JO 

- 

a\    /  b  \ 

5 

_.     1 

/ 1             1             t             >             1             1             1             !            1         ._i^. L 

ZO       24-       £8       32       36       40       44       48        5S        56       60       64 

Fig.  26.  —  The  body  width  of  a  population  of  the  protozoan  Paramecium, 
showing  a  polygon  with  two  modes.  A,  Parajnecium  aurelia.  B, 
Paramecium  caudatum.    After  Jennings. 

for  example,  in  measuring  the  body  width  of  a  popu- 
lation of  the  protozoan  Paramecium  (Fig.  26). 

1 "  Variation  in  Urosalpinx."    Walter.     Amer.  Nat.  1910,  Vol.  XLIV, 
pp.  577-594. 


VARIATION 


49 


It  was  subsequently  found  that  the  two  modes  in 
this  polygon  were  due  to  the  fact  that  the  material 
in  question  was  a  mixture  of  two  closely  related 
species,  Paramecium  aurelia  and  Paramecium  cauda- 
tum,  the  individuals  of  which  arranged  themselves 
around  their  own  mean  in  each  instance. 


Number  q\ 
leaves 


\z 
w 

;o 

9 
8 
7 
6 
5 
4 
3 
2 
1 


Number  ot        J  3  H  15        16 

ribs 


17       18       19       10       EI 


Fig.  27.  —  The  ribs  of  leaves  from  two  beech  trees.  When  put  together 
they  form  a  polygon  which  does  not  reveal  its  double  origin.  From 
data  by  Pearson. 

Although  such  an  explanation  does  not  always 
turn  out  to  be  the  right  one,  the  biometrician  is 
led  to  suspect  when  a  two  or  more  moded  polygon 
appears  that  he  is  dealing  with  a  mixture  of  more 
than  one  kind  of  material,  each  of  which  fluctuates 
around  its  own  average. 

Heterogeneous  material,  it  should  be  noted,  does 
not  always  give  a  bimodal  curve.  For  example, 
if   Pearson's   two   lots    of   beech    leaves   mentioned 


E 


50  GENETICS 

above  are  mixed  together,  they  form  a  regular 
series  from  the  inspection  of  which  no  one  could  infer 
their  double  origin.    (See  the  heavy  line  in  Figure  27.) 

c.  Shew  Polygons 

The  direction  in  which  variations  are  tending 
may  sometimes  be  determined  by  the  statistical 
method.  As  an  illustration  of  this  may  be  cited  the 
number  of  ray  florets  on  1000  white  daisies  {Chrys- 
anthemum leucanthemum) ,  500  of  which  were  col- 
lected at  random  by  the  writer  from  a  small  patch 
in  a  swampy  meadow  in  northern  Vermont,  while 
the  other  500  were  selected  in  the  same  random 
manner  upon  the  same  day  from  a  dry  hillside  pas- 
ture hardly  more  than  a  stone's  throw  distant. 
Among  these  two  lots  of  daisies  the  number  of  ray 
florets  varies  from  twelve  to  thirty-eight  and  their 
frequency  polygons,  as  shown  in  Figure  28,  form 
what  are  termed  "skew polygons,"  because  the  mode 
in  each  case  lies  considerably  to  one  side  of  the  arith- 
metical mean. 

It  will  be  seen  that  lot  A  from  the  swampy  meadow, 
which  in  spite  of  the  greater  fertility  of  the  soil  and 
the  unquestionably  greater  luxuriance  of  the  plants 
themselves,  produced  heads  with  fewer  florets, 
fluctuates  around  the  number  21,  while  the  dry 
pasture  population  B,  characterized  by  blossoms 
which  were  in  general  noticeably  smaller,  fluctuates 
around  the  number  34. 


VARIATION 


51 


The  habitats  of  the  two  lots  were  so  near  together, 
however,  that  there  was  probably  a  considerable 
intermixture  of  the  two  types,  as  shown  by  the 
tendency  of  each  polygon  to  produce  a  second  mode. 


90 
85 
80 
15 
70 
65 
60 
55 
50 
45 
40 

35 
30 

25 

20 

;5 

10 
5 

Fig, 


Kv!''® 


12  13  H  15  16  17  18  19  EO  Z1     It    23  24  25  26  27  28  29  30  31   32  33  34  JS  36  37  38  39 


.  28. — Variation  in  the  ray  florets  of  the  white  daisy  (Chrysanthe- 
mum leucanthemum) .  A,  from  a  swampy  meadow.  B,  from  a  hillside 
pasture  near  by.  Both  the  polygons  are  "  skew  "  because  in  each  case 
there  is  an  admixture  of  the  other  type.  The  distinction  between 
the  two  types  is  due  to  heredity  rather  than  to  environment. 


Thus  the  A  polygon  shows  that  there  is  an  increasing 
tendency  or  variability  in  the  twenty-one  floret 
type  toward  the  thirty-four  floret  type,  due  probably 
in  this  particular  instance  to  invasion  resulting 
from  the  proximity  of  the  B  colony. 


52  GENETICS 

8.    Graduated,  and  Integral  Variations 

It  is  comparatively  simple  to  treat  statistically 
integral  variations,  illustrations  of  which  have  been 
given  in  the  case  of  beech-leaf  ribs,  starfish  rays,  and 
daisy  florets,  all  of  which  are  characters  that  can  be 
readily  counted.  In  the  same  way  any  measurable 
character,  such  as  the  size  of  snail  shells,  may  fall 
into  easily  limited  groups,  as,  for  example,  10  to 
11  mm.,  11  to  12  mm.,  12  to  13  mm.,  etc.  It  is 
somewhat  more  difficult  to  classify  variations  when 
color  or  pattern  is  the  character  in  question,  since  it 
then  becomes  necessary  to  define  certain  arbitrary 
limits  for  each  class  of  the  series  within  which  to 
group  the  individual  variants. 

Tower,  in  his  famous  researches  on  potato-beetles, 
encountered  variations  in  the  pigmentation  of  the 
pronotum  all  the  way  from  entire  absence  of  color 
to  complete  pigmentation.  By  cutting  up  this 
continuous  series  of  variations  into  arbitrary  groups 
of  equal  extent,  however,  it  was  quite  possible  to 
arrange  the  data  so  that  they  could  be  statistically 
treated  just  as  conveniently  as  the  integral  variations 
mentioned  above.  Groups  or  classes  of  this  kind 
are  termed  graduated  variations. 

9.   The  Causes  of  Variation 

With  respect  to  the  causes  of  variation  authori- 
tative biologists  have  taken  different  points  of  view. 

a.  Darwin  considered  variations  as  axiomatic. 
An  axiom  is  self-evident,  requiring  no  explanation. 


VARIATION 


53 


The  absence  of  variations  in  organisms  rather  than 
the  occurrence  of  variations  is,  from  this  point  of 
view,  the  phenomenon  requiring  an  explanation. 
Although  Darwin  himself  spent  some  time  in  point- 
ing out  the  universal  occurrence  of  variability,  he 
accepted  it  as  a  primary  fact  and  proceeded  from  it 
as  a  starting  point  without  attempting  to  seek  its 
causes. 

b.  Lamarck  and  his  followers  have  regarded  the 
causes  of  variation  either  as  extrinsic,  that  is,  refer- 
able to  external  factors  making  up  the  environment  of 
the  organism,  or  as  intrinsic  or  physiological,  that 
is,  based  upon  the  efforts  which  an  organism  puts 
forth  to  fit  into  its  particular  environment  success- 
fully.    The  causes  of  variation  are  to  be  sought  ac- 


t 


■oO 


-L. 


-L. 


Af. 


M, 


t o/ 

50      35       '^0       45       50        55       60       65        70       75        80       85       90       95       100  /o 

< Ratio     of     height  of  head   to   length  of  shell  > 

Fig.  29.  —  Schematic  curve  of  the  head  height  of  Hyalodaphnia  under 
various  conditions  of  nourishment.    Adapted  from  Woltereck. 


cording  to  the  Lamarckian  school,  in  the  "environ- 
ment" and  "training"  sides  of  the  triangle  of  life 
rather  than  in  the  "heritage"  side  (Fig.  l)c 

For  example,  Woltereck,  by  controlling  the  single 


54 


GENETICS 


extrinsic  factor  of  food  supply,  was  able  to  modify 
the  height  of  the  *'head"  of  the  microscopic  fresh- 
water crustacean,  Hyalodaphnia,  in  the  remarkable 
manner  indicated  in  Figure  29.     When  poor  food 


Number  of 
Flowers 


Number  cf  L 
oUaens  lo 


6      5 


10     9      6 


10      9      8 


Fig.  30.  —  Variations  in  the  number  of  stamens  in  the  flowers  of  the  "  live- 
for-ever"  (Sedum  spectabile)  under  various  controlled  conditions. 
For  detailed  description,  see  text.    After  Klebs. 


was  supplied,  the  percentage  of  the  head  height  to 
that  of  the  body  averaged  hardly  forty,  while  with 
rich  food  it  was  increased  to  over  ninety. 

Similarly  Klebs  succeeded  in  changing  at  will  the 
number  of  stamens  in  the  common  "  live-for-ever," 
Sedum  spectabile,  by  manipulating  the  environment 
in  which  the  plants  were  kept.  Some  of  his  results 
are  shown  in  Figure  30.  Polygon  A  combines  the 
data  for  4260  flowers  which  were  raised  in  well-fer- 
tilized dry  soil  under  bright  light ;  polygon  B  repre- 
sents 4000  flowers  grown  in  a  moist  greenhouse 
under  red  light ;  and  polygon  C  includes  4390  flowers 


VARIATION  55 

from  well-fertilized  soil  in  moist  hotbed  conditions 
under  a  weak  light. 

c.  Weismann,  on  the  contrary,  believes  that  the 
causes  of  variation,  at  least  of  heritable  variations, 
are  intrinsic  or  inborn  in  the  germplasm.  His  con- 
ception of  sexual  reproduction  is  that  it  is  a  device 
for  doubling  the  possible  variations  in  the  offspring 
by  the  mingling  of  two  strains  of  germplasm  {am- 
phimixis).  By  far  the  greater  number  of  observa- 
tions recorded  go  to  substantiate  this  theory. 

Tower  found  among  his  potato-beetles,  for  exam- 
ple, that  two  strains  reared  in  the  same  environment 
showed  striking  differences  in  variation,  —  a  fact 
necessarily  due  to  intrinsic  rather  than  to  extrinsic 
factors.     Similar  cases  may  be  recalled  by  any  one. 

d.  Lastly,  Bateson,  whose  work  "On  Materials 
for  the  Study  of  Variation"  already  cited  is  a  classic, 
takes  the  agnostic  attitude  that  it  is  rather  futile 
to  guess  at  the  causes  of  variation  before  the  facts 
are  well  in  hand.  He  consequently  discourages  such 
attempts  by  saying  :  "Inquiry  into  the  causes  of 
variation  is,  in  my  judgment,  premature." 

In  conclusion,  the  words  of  Darwin  written  half 
a  century  ago  —  "Our  ignorance  of  the  laws  of 
variation  is  profound"  —  may  still  be  appropriately 
quoted,  notwithstanding  the  fact  that  in  biometry 
we  have  at  least  an  excellent  analytical  method  by 
means  of  which  considerable  insight  into  variation 
is  being  gained. 


CHAPTER  IV 

MUTATION 

1.   The  Mutation  Theory 

Among  the  possible  kinds  of  variation  already 
hinted  at  are  so-called  mutations  which  are  clearly 
defined  from  the  fluctuating  variations  to  which 
reference  has  just  been  made. 

Darwin  was  fully  aware  of  the  existence  of  muta- 
tions or  "sports"  and  incidentally  gave  time  to 
their  consideration,  but  the  great  task  which  he 
accomplished  in  such  a  masterly  manner  was  to 
overthrow  the  widespread  and  deep-seated  belief 
of  his  day  in  a  sudden  special  creation  of  distinct 
species.  To  this  end  he  marshaled  evidence  in 
support  of  the  gradual  transition  of  one  species  into 
another,  emphasizing  fluctuations  rather  than  muta- 
tions which  seemed  to  him  to  play  a  minor  role  in 
the  origin  of  species. 

It  remained  for  the  Dutch  botanist  Hugo  de  Vries 
to  analyze  the  character  of  mutations.  There  is 
something  distinctly  suggestive  of  Darwin's  method 
in  the  fact  that  de  Vries  worked  in  silence  for  twenty 
years  before  he  gave  to  the  world  the  *'Mutations- 
theorie"  with  which  his  name  will  forever  be  con- 
nected. 

56 


MUTATION  51 

2.   Mutation  and  Fluctuation 

A  mutation  is  something  qualitatively  new  that 
appears  abruptly  without  transitions  and  which  breeds 
true  from  the  very  first.  To  use  the  musician's 
phraseology,  it  is  not  a  variation  elaborated  upon  an 
old  theme,  which  would  correspond  to  a  fluctuating 
variation,  but  it  is  an  entirely  new  theme.  The 
difference  between  mutations  and  fluctuating  varia- 
tions is  generally  not  one  of  quantity  or  magnitude, 
although  it  sometimes  may  be  so,  —  since  muta- 
tions are  often  much  smaller  than  fluctuations. 
Mutations  are  discontinuous  in  the  same  sense  that 
chemical  combinations,  such  as  carbon  monoxide 
(CO)  and  carbon  dioxide  (CO2),  are  discontinuous, 
but  the  leap  from  one  to  the  other  may  be  so  small 
that  frequently  it  is  diflicult  to  ascertain  by  inspec- 
tion alone  whether  the  difference  is  due  to  a  mutation 
or  a  fluctuation.  The  test  comes  in  breeding,  for  the 
progeny  of  a  fluctuation  will  vary  around  the  old 
average  of  the  parental  generation,  while  the  progeny 
of  a  mutation  will  vary  around  a  new  average,  set 
by  the  mutation  itself. 

When  a  series  of  mutations  is  treated  statistically, 
it  does  not  arrange  in  frequency  polygons  as  readily 
as  a  series  of  fluctuations  do.  The  latter  mass 
around  the  average  standard  according  to  the  laws 
of  chance  much  in  the  same  way  that  a  hundred  shots 
by  a  good  marksman  may  center  around  a  bull's- 
eye.  Mutations  never  act  in  this  way.  They  find 
no  correspondence  even  with  wild  shots  at  the  bull's- 


58  GENETICS 

eye.      They  are  shots  directed  at   a   different  target 
altogether. 

To  the  student  of  heredity  there  are  two  distinc- 
tions of  prime  importance  with  respect  to  mutations. 
First,  that  they  usually  appear  full-fledged  without 
preparatory  stages,  and  second,  that  they  breed  true 
from  the  start.  Fluctuations,  on  the  contrary,  ordin- 
arily  "revert"  to  the  parental  type  in  subsequent  gen- 
erations. The  great  practical  importance  to  the 
breeder  of  a  knowledge  of  these  heritable  mutations 
is  at  once  apparent. 

3.   Freaks 

A  further  distinction  should  be  made  between 
mutations  and  so-called  freaks  or  monstrosities, 
namely,  that  the  former  breed  true,  while  the  latter 
do  not.  A  human  physical  deformity,  such  as  a 
club-foot,  for  example,  or  a  humped  back,  is  not  a 
mutation,  because  it  does  not  reappear  as  a  heritable 
character.  Variations  of  this  kind  are  not  predeter- 
mined in  the  germplasm,  but  are  usually  instances  of 
something  that  went  wrong  during  the  development 
of  the  individual  somatoplasm. 

Thus,  among  normally  "right-handed"  snails  "left- 
handed"  individuals  have  occasionally  been  dis- 
covered which,  when  bred,  were  found  to  produce 
all  normal  "right-handed"  progeny.  They  are 
therefore  not  mutations  at  all,  but  freaks  or  mon- 
strosities due  probably  to  some  unusual  occurrence 
early  in  the  cleavage  stages  of  the  embryo. 


MUTATION  59 

4.    Kinds  of  Mutation 

De  Vries  has  classified  mutations  according  to 
their  component  units  into  three  categories:  pro- 
gressive, regressive,  and  degressive. 

Progressive  mutations  are  signaHzed  by  the  addi- 
tion of  a  new  character  to  the  sum  of  complex  char- 
acters making  up  the  individual.  If  rumor  may  be 
believed,  iViine  Boleyn,  the  second  in  the  interesting 
series  of  wives  of  Henry  VIII,  was  a  progressive  mu- 
tant with  respect  to  at  least  three  characters,  for  she 
is  said  to  have  been  possessed  of  an  extra  finger  on 
each  hand,  supernumerary  mammae,  and  extra  teeth. 
Evidences  that  each  of  these  three  characters  occur 
as  heritable  mutations  is  presented  in  Davenport's 
*' Heredity  in  Relation  to  Eugenics." 

Regressive  mutations  are  characterized  by  the 
dropping  out  of  something.  Thus  albinism  is  caused 
by  the  absence  of  pigment  or  color.  Albinic  mutants 
which  breed  true  are  well  known,  particularly  among 
mammals,  such  as  rats,  mice,  rabbits,  cats,  guinea- 
pigs,  and  even  man  himself. 

Degressive  mutations  include  cases  of  the  return 
of  a  character  which  was  formerly  present  in  the 
past  history  of  the  race,  but  which  has  for  generations 
been  absent  or  latent.  Castle's  four-toed  race  of 
guinea-pigs  furnishes  an  example  of  this  class  of 
mutations.  In  1906  Professor  Castle  discovered  a 
newly  born  guinea-pig  in  one  of  his  pens  with  four 
toes  on  each  hind  foot,  from  which  he  has  successfully 
established  a  four-toed  race.     The  hypothetical  an- 


60  GENETICS 

cestor  of  the  rodents  probably  had  five  toes  on  each 
foot,  but  the  normal  number  in  modern  guinea-pigs 
is  four  on  each  of  the  front  feet  and  three  on  the 
hind  feet.  The  individual  from  which  Castle  has 
bred  a  four-toed  race  exhibited  a  degressive  muta- 
tion, tending  toward  the  ancestral  type. 

5.    Species  and  Varieties 

The  doctors  have  always  disagreed  regarding  a 
definition  of  species.  What  determines  the  exclusive 
boundaries  that  shall  isolate  from  their  fellows  any 
particular  group  of  animals  or  plants  has  long  been 
a  mooted  question,  and  still  remains  so. 

The  Linnsean  concept  of  a  species  was  that  of  an 
exclusive  caste  of  individuals,  inflexibly  demarked, 
over  w^hose  high  barriers  no  nondescript  tramps 
would  dare  attempt  to  climb.  When  an  entomolo- 
gist of  the  old  Linnsean  school  encountered  an  insect 
which  did  not  conform  to  the  morphological  tradi- 
tions of  its  fellows,  the  frequent  fate  of  such  a  non- 
conformist was  to  perish  under  the  boot-heel  rather 
than  to  find  sanctuary  in  the  cabinet  of  the  pre- 
served. Since  it  was  an  exception,  and  a  violator 
of  the  divine  law  of  the  fixity  of  species,  it  deserved 
to  be  annihilated!  Those  were  hard  days  both  for 
heretics  and  for  mutations. 

The  method  of  the  older  school  of  systematists 
may  be  described  as  one  which  emphasized  differences 
and  put  up  barriers  that  should  keep  the  unlike 
apart,  at  the  same  time  allowing  only  "birds  of  a 
feather  to  flock  together."     It  was  a  brave  and  sue- 


MUTATION  61 

cessful  attempt  to  bring  order  out  of  chaos  by  classi- 
fying the  living  world,  and  it  served  its  purpose  well 
until  Darwin's  idea  of  half  a  century  ago,  that  the 
origin  of  all  species  is  from  preceding  species,  put  an 
entirely  new  face  upon  the  whole  matter.  Organ- 
isms of  different  species  were  found  to  be  related  to 
one  another,  and  even  man  could  no  longer  escape 
acknowledging  his  poor  animal  relations.  As  a 
consequence,  likenesses  rather  than  differences  there- 
after claimed  the  most  attention. 

During  the  reconstruction  of  phylogenetic  trees, 
which  seized  the  imagination  and  became  the  prin- 
cipal business  of  biologists  as  soon  as  the  "  Origin  of 
Species  "  was  made  common  property,  the  crotched 
sticks  in  the  woodpile  of  organisms,  that  had  hitherto 
been  largely  discarded,  were  most  eagerly  sought  after. 
It  was  just  these  scraggly  sticks,  that  were  neither 
trunk  nor  limb-wood  but  combinations  of  both, 
which  told  the  story  of  continuity  and  were  indis- 
pensable in  building  up  a  reunited  whole. 

As  the  analysis  of  the  living  world  gradually  came 
to  shift  from  species  to  individuals,  it  was  shown 
that  individuals  may  be  regarded  simply  as  ag- 
gregates of  unit  characters  which  may  combine  so 
variously  that  it  becomes  more  and  more  diffi- 
cult to  maintain  constant  barriers  of  any  kind  be- 
tween the  groups  of  individuals  arbitrarily  called 
species. 

The  old  species  of  the  systematist,  upon  analysis 
into  their  respective  unit  characters,  dissolve  into 
numerous  "elementary  species"  and  "varieties"  dif- 


62 


GENETICS 


fering   from  species  perhaps   only  by   the  addition 
or  subtraction  of  a  single  character,  and  thus  the 


Fig.  31. — Diagram  to  illustrate  various  ideas  about  "species."  Under 
Species  A  are  represented  two  groups  of  individuals  which  are  near 
enough  alike  to  be  placed  within  a  single  species,  but  which  are  suffi- 
ciently unlike  each  other  to  constitute  the  "  sub-species  "  or  "varie- 
ties "  of  Darwin.  Under  Species  B  are  various  groups  of  individuals 
distinguished  from  each  other  by  the  addition  or  loss  of  one  or  more 
characters.  These  groups  represent  the  "elementary  species"  and 
"varieties"  of  de  Vries.  'The  "barrier  of  Linnaeus"  attempted  to 
separate  species  absolutely  from  each  other.  Darwin  sought  to  find 
loopholes  in  this  barrier.  To-day  attention  is  directed  rather  to  the 
relation  between  individuals  than  to  the  boundaries  between  species. 

possibilities  of  analytical  classification  have  become 
almost  limitless. 

An  elementary  species,  according  to  de  Vries,  is  a 
progressive  mutation  differing  from  the  type  species 


MUTATION  63 

by  the  addition  of  at  least  a  single  character,  while 
varieties  are  regressive  mutations  distinguished  from 
the  parent  type  by  the  loss  of  at  least  one  character. 
Both  breed  true  to  their  respective  modifications. 

These  different  concepts  of  what  constitutes  a 
species,  illustrated  diagrammatically  in  Figure  31, 
pave  the  way  for  a  better  understanding  of  muta- 
tions in  connection  with  heredity. 

6.   Plant  Mutations  found  in  Nature 

The  oldest  known  authenticated  case  of  a  plant 
mutation  is  the  often  cited  instance  of  the  *' fringed 
celandine,"  Chelidonium  laciniatum,  which  made  its 
appearance  in  the  garden  of  the  Heidelberg  apothe- 
cary Sprengel  in  1590  among  plants  of  the  "greater 
celandine,"  Chelidonium  majus.  The  fringed  cel- 
andine bred  true  at  once  and  is  now  a  widespread 
and  well-known  species. 

The  purple  beech  has  appeared  historically  as  a 
mutant  among  ordinary  beeches  upon  at  least  three 
occasions  in  widely  separated  localities,  and  it  has 
always  given  rise  to  a  constant  progeny. 

The  "Shirley  poppy,"  notable  for  its  remarkable 
range  of  color,  originated  from  a  single  plant  of  the 
small  red  poppy,  Papaver  rhoeas,  which  is  commonly 
found  in  English  cornfields. 

Instances  are  known  of  double  flowers  among 
roses,  azaleas,  stocks,  carnations,  primroses,  petunias, 
etc.,  arising  from  single  flowering  plants,  the  seeds  of 
which  in  turn  produce  double  flowers. 


64  GENETICS 

7.   Lamarck's  Evening  Primrose 

The  most  widely  known  plant  mutations  are  the 
progeny  of  Lamarck's  evening  primrose,  Oenothera 
lamarckiana,  because  it  was  these  plants  that  led 
de  Vries  to  formulate  his  mutation  theory. 

It  is  believed  by  botanists  in  general  that  this 
plant  is  a  native  of  the  southern  United  States,  al- 
though it  is  now,  so  far  as  is  knowm,  extinct  as  a 
wild  species  in  America,  and  native  specimens  are 
included  in  but  few  American  herbaria. 

It  was  exported  to  London  as  a  garden  plant  about 
1860,  and  from  thence  it  spread  to  the  continent, 
where,  escaping  from  gardens,  it  became  wild  in  at 
least  one  locality  neg.r  Hilversum,  a  few  miles  from 
Amsterdam.  Here,  in  an  abandoned  potato  field, 
it  fell  under  the  seeing  eye  of  Hugo  de  Vries  in  1885, 
and  now  both  botanist  and  primrose  are  famous. 

De  Vries  found  among  these  escaped  plants  not 
only  0.  lamarckiana,  but  also  two  other  kinds 
or  mutants,  0.  brevistylis,  characterized  by  short- 
styled  flowers,  and  0.  Ice vi folia,  w^iich  has  smooth 
leaves.  These  tw^o  were  entirely  new  species  hitherto 
unknown  at  the  great  botanical  clearing-houses  of 
Paris,  Leyden,  and  the  Kew  Gardens. 

Since  the  seeds  of  the  (Enothera  are  produced  by 
self-fertilized  flowers,  de  Vries  felt  safe  in  regard- 
ing these  plants  as  mutants  rather  than  hybrids, 
and  he  continued  to  study  them  with  especial  care. 
Transplanting  the  mutants  along  with  representa- 
tives of  0.   lamarckiana  to  his  private  gardens  in 


MUTATION 


65 


Amsterdam,  where  it  was  possible  to  maintain  them 
in  normal  healthy  condition,  de  Vries  was  able  to 
follow  their  individual  histories  with  certainty. 

He  found  that,  out  of  54,343  plants  of  the  species 
0.  lamarckiana  grown  during  eight  years,  there  ap- 
peared 837  mutants  comprising  seven  different  ele- 
mentary species,  all  of  which,  with  the  exception  of 
0.  scintillans,  bred  true.     See  table. 

Mutants  of  (Enothera  lamarckiana 


Generation 

<! 
o 

O 

Q 

3 
< 

•< 

z 

o 
ij 
n 
O 

CO 

M 

a 

K 

m 

< 
iz; 
< 

« 

< 

.-1 

w 
< 

< 

< 

35 

< 
.J 

U 

Total 

I 

1886-7 

9 

II 

1888-9 

i5,ooa 

5 

5 

III 

1890-1 

1 

10,000 

3 

3 

IV 

1895 

1 

15 

176 

8 

14,000 

60 

73 

1 

V 

1896 

25 

135 

20 

8,000 

49 

142 

6 

VI 

1897 

11 

29 

3 

1,800 

9 

5 

1 

VII 

1898 

9 

3,000 

11 

VIII 

1899 

5 

1 

1,700 

21 

1 

1 

56 

350 

32 

53,509 

158 

229 

8 

54,343 

Some  explanatory  comment  on  this  table  may  be 
of  value. 

The  seeds  in  each  generation  were  self-fertilized 
lamarckiana. 

The  mutant  gigas  occurred  once,  in  1895.  From 
the  seeds  of  this  one  plant  were  produced  450  true 
gigas  offspring  in  the  first  year,  and  the  strain  con- 
tinues to  breed  true. 

Albida  was  first  noted  in  1895,  but  de  Vries  remem- 


66  GENETICS 

bered  having  seen  it  before  and  dismissing  it  as 
pathological.  Because  of  its  poverty  in  chlorophyll 
it  is  a  mutant  which  probably  would  not  maintain 
itself  successfully  in  nature,  but  it  breeds  constant 
under  cultivation. 

Ohlonga  always  bred  true  with  the  exception  of 
throwing  an  alhida  in  1895  and  a  ruhrinervis  in  1899. 

Of  ruhrinervis  over  2000  invariably  bred  true, 
while  nanella  bred  true  in  over  20,000  offspring,  with 
but  three  exceptions  when  ohlonga  characters  appeared. 

Lata,  since  it  produces  only  female  flowers  and  so 
cannot  be  self-fertilized,  had  constantly  to  be  crossed 
back  with  the  parent  lamarclciana,  when  it  produced 
from  15  to  20  per  cent  lata  and  80  to  85  per  cent 
lamarckiana. 

Finally,  scintillans  which  appeared  at  three  separate 
times  proved  constant  only  in  its  inconstancy  be- 
cause it  invariably  produces  a  heterogeneous  progeny. 
The  1895  plant  gave  53  per  cent  lamarckiana,  35 
per  cent  scintillans,  10  per  cent  ohlonga,  and  1  per 
cent  lata.  One  of  the  1896  plants  gave  51  per  cent 
lamarckiana,  39  per  cent  scintillans,  8  per  cent 
ohlonga,  1  per  cent  lata,  and  1  per  cent  nanella,  while 
another  1896  plant  gave  only  8  per  cent  lamarckiana, 
but  69  per  cent  scintillans,  21  per  cent  ohlonga,  and 
2  per  cent  of  nanella  and  lata  together. 

These  seven  elementary  species  are  distinguished 
from  each  other  bv  features  which  are  unmistakable 
even  to  the  uninitiated.  The  old-time  systematist 
would  undoubtedly  have  regarded  them  as  distinct 
species. 


MUTATION  67 

De  Vries'  experiments  and  observations  have  been 
repeated  on  a  large  scale  and  extended,  notably  by 
MacDougall  in  the  New  York  Botanical  Gardens 
and  by  Shull  at  the  Carnegie  Institution  for  Ex- 
perimental Evolution,  Cold  Spring  Harbor,  Long 
Island,  and  his  conclusions  have  been  confirmed  in 
all  essential  points.  The  mutability  of  0.  lamarcki- 
ana  is  as  unmistakable  and  as  diverse  in  America  as 
it  is  in  Holland. 

A  parallel  case  of  a  plant  caught  in  the  act  of  giving 
rise  to  mutations  is  that  of  the  roadside  weed  Lychnis, 
reported  by  Shull,  and  the  phenomenon  is  probably 
by  no  means  as  unusual  as  is  generally  believed. 
The  chief  reason  why  such  definite  examples  of  mu- 
tation are  so  infrequently  noted  and  recorded  is 
because  the  attention  of  the  investigator  has  generally 
been  directed,  not  to  them,  but  to  gradual  fluctuating 
variations  which,  according  to  Darwin's  conception, 
furnish  the  material  for  the  operation  of  natural 
selection.  Mutations  are  doubtless  much  more  com- 
mon than  has  been  generally  supposed,  and  it  is  likely 
that  they  will  receive  more  attention  in  the  future 
than  they  have  in  the  past.  De  Vries  rather  pointedly 
says:  "The  theory  of  mutations  is  a  starting-point 
for  direct  investigation,  while  the  general  belief  in 
slow  changes  has  held  back  science  from  such  in- 
vestigations during  half  a  century." 

8.   Some  Mutations  among  Animals 

In  1791  a  Massachusetts  farmer,  by  name  Seth 
Wright,  found  in  his  flock  of  sheep  a  male  lamb  with 


68  GENETICS 

long,  sagging  back  and  short,  bent  legs  resembling 
somewhat  a  German  dachshund.  With  unusual 
foresight  he  carefully  brought  up  this  strange  lamb 
because  it  was  an  animal  that  could  not  jump  fences. 
It  occurred  to  this  hard-headed  Yankee  that  it  would 
be  much  easier  to  get  together  a  flock  of  short,  bow- 
legged  sheep,  unable  to  negotiate  anything  but  a  low 
hurdle,  than  to  labor  hard  at  building  high  fences. 
So  it  came  about  that  this  mutating  lamb,  in  the  hands 
of  a  man  who  appreciated  labor-saving  devices,  be- 
came the  ancestor  of  the  Ancon  breed  of  sheep. 
Later  on  this  breed  gave  place  in  public  favor  to  an- 
other mutant,  the  Merino,  which  produces  a  superior 
grade  of  wool. 

Hornless  cattle  suffer  fewer  injuries  from  one  an- 
other than  horned  cattle.  It  has  consequently  be- 
come quite  a  general  practice  among  farmers  to 
"dehorn"  their  stock  surgically.  It  is  an  obvious  ad- 
vantage to  have  cattle  born  hornless,  and  many  breeds 
having  this  character  are  now  established.  In  1889 
a  mutant  among  horned  stock  appeared  at  Atkinson, 
Kansas,  in  the  form  of  a  hornless  Hereford.  From 
this  mutant  has  descended  the  well-established  race 
of  polled  Hereford  cattle,  constituting  a  bovine  aristoc- 
racy^ with  registry  books  and  blue  blood  all  their  own. 

Taillessness  in  cats,  dogs  and  poultry,  as  well  as 
hairlessness  in  cattle,  dogs,  mice  and  horses,  are 
further  instances  of  mutations. 

Davenport,^  writing  of  his  experiments  with  poultry, 

^  Davenport,  C.  B..  1909.  "  Inheritance  of  Characteristics  in  Domestic 
Fowl."     Carnegie  Institution  of  Washington,  Pubhcation  No.  121. 


MUTATION  69 

says:  "During  the  past  four  years  I  have  handled 
and  described  over  10,000  poultry  of  known  ancestry. 
Of  striking  new  characters  I  have  observed  many, 
some  incompatible  with  normal  existence;  others  in 
no  way  unfitting  the  individual  for  continued  life. 
In  the  egg  unhatched  I  have  obtained  Siamese  twins, 
pug  jaws,  and  chicks  with  thigh  bones  absent.  There 
have  been  reared  chicks  with  toes  grown  together 
by  a  web,  without  toenails  or  with  two  toenails  to 
a  toe ;  with  five,  six,  seven,  or  three  toes ;  with  one 
wing  or  both  lacking ;  with  two  pairs  of  spurs  ;  with- 
out oil-gland  or  tail ;  with  neck  devoid  of  feathers ; 
with  cerebral  hernia  and  a  great  crest ;  with  feather 
shaft  recurved,  with  barbs  twisted  and  dichoto- 
mously  branched  or  lacking  altogether.  Of  comb 
alone  I  have  a  score  of  forms.  xVll  of  these  characters 
have  been  offered  to  me  without  the  least  effort  or 
conscious  selection  on  my  part,  and  each  appeared  in 
the  first  generation  as  well-developed  peculiarities, 
and  in  so  far  as  their  inheritance  was  witnessed,  each 
refused  to  blend  when  mated  with  a  dissimilar  form." 
Bateson  (1894),  in  his  "Materials  for  the  Study  of 
Variation,"  gives  a  detailed  list  of  886  cases  of  "dis- 
continuous variations"  among  animals,  many  of  which 
doubtless  belong  to  the  category  of  mutations,  al- 
though several  must  be  placed  in  the  non-inherit- 
able class  of  "freaks." 

9.   Possible  Explanations  of  IVIutation 

It  is  apparent  that  the  causes  of  mutations,  since 
they  occur  regardless  of  the  environment,  are  ])rob- 


70 


GENETICS 


ably  of  an  intrinsic  or  germinal  nature.  Evening 
primroses  display  the  same  mutants  whether  in 
Holland  or  America,  in  a  wild  state  or  under  culti- 
vation. Mutations,  like  poets,  are  born,  not  made. 
It  has  been  suggested  by  Standfuss  that  species 
may  go  through  the  same  kind  of  a  life-cycle  that  in- 
dividuals do,  only 
'^  taking      infinitely 

more  time  to  do 
it.  As  shown  in 
Figure  32,  they 
are  born  of  other 
species  and  enter 
the  prodigious 
growth  period  of 
infancy  and  youth, 
both  of  which  are 
characterized  by 
much  fluctuation. 
With  maturity  they  gradually  become  comparatively 
stable  until  the  reproductive  period  is  reached,  when 
they  throw  off  their  progeny,  as  on  a  tangent.  They 
finally  pass  into  the  excessively  differentiated  period 
of  old  age,  from  which  there  is  no  recall,  although 
it  approaches  in  many  features  the  infantile  condition, 
and  end  in  death  or  extinction.  This  cycle  is  repeat- 
edly illustrated  by  phylogenetic  lines  of  fossil  forms 
which  have  long  since  become  extinct. 

Beecher  has  pointed  out  that,  in  paleontological 
times  just  before  they  became  extinct,  species  often 
underwent    extreme    specialization    in    the    form    of 


Fig.  32.  —  Diagram  of  the  relation  of  repro 
duction  to  the  Hfe-cycle. 


MUTATION  71 

fantastic  shapes,  an  excessive  number  of  spines  or 
elaborate  sculpturings  on  the  shells  as  seen  among 
the  ammonites,  belemnites,  and  trilobites,  or  of 
gigantic  size  as  in  the  dinosaurs,  plesiosaurs,  and 
theromorphs.  All  of  these  facts  indicate  a  species- 
cycle  in  which  these  abnormal  features  were  the  un- 
mistakable signs  of  old  age. 

The  reproductive  period  of  a  species  when  mutants 
are  being  thrown  off,  as  of  an  individual,  may  ex- 
tend over  a  considerable  period  of  the  whole  cycle, 
or  it  may  be  confined  to  a  relatively  small  segment. 
It  is  possible  that  in  the  evening  primrose  de  Vries 
may  have  caught  a  plant  passing  through  the  crucial 
period  of  species-reproduction. 

Another  reason  why  so  few  mutations  have  as  yet 
been  seen,  is  because  the  majority  of  organisms  are 
not,  during  the  short  span  of  human  observation, 
in  the  reproductive  part  of  their  cycles.  When  it  is 
remembered  that  accurate  observation  with  this 
object  in  view  has  extended  over  only  a  brief  period, 
insignificant  in  comparison  with  the  vast  geologic 
stretches  of  time  concerned  in  species-building,  the 
marvel  is  that  so  much,  rather  than  that  so  little,  has 
been  seen. 

A  further  suggestion  in  connection  with  the  pos- 
sible sources  of  mutation  is  that  mutations  may  be 
the  results  of  hybridization,  appearing  as  INIendelian 
recessives  after  crossing.  As  a  matter  of  fact,  Spron- 
gel's  Chelidonium  laciniatum,  already  cited,  when 
crossed  with  Chelidonium  ma  jus,  behaves  according  to 
such   an   expectation.     This   phase   of   the   question, 


72  GENETICS 

however,  may  be  more  suitably  considered  later  after 
what  is  meant  by  a  "Mendelian  recessive"  has  been 
made  clear. 

It  is  extremely  doubtful,  however,  whether  any 
recombination  of  parental  characters  in  a  hybrid  may 
properly  be  called  a  mutation,  since  no  character 
strictly  new  is  thus  produced. 

10.  A  Summary  of  the  Mutation  Theory 

•  The  main  features  of  the  mutation  theory  of  de 
Vries  may  be  indicated  as  follows  :  — 

a.  New  species  arise  abruptly  regardless  of  environ- 
ment without  transitional  forms,  and  at  present  they 
are  not  known  to  arise  in  any  other  way. 

h.  New  forms  arise  as  unusual  deviations  from  the 
parent  form,  which  itself  remains  unchanged,  al- 
though it  may  repeatedly  give  rise  to  similar  devia- 
tions. 

c.  New  mutations  are,  from  the  first,  constant,  that 
is,  they  produce  their  like.  They  do  not  become 
gradually  established  as  t^e  result  of  natural  selection. 

d.  Among  mutations  there  may  occur  forms 
characterized  by  the  addition  of  something  new,  — 
progressive  elementary  species,  —  as  well  as  forms 
lacking  something  present  in  the  parental  type,  — 
regressive  varieties. 

e.  The  same  mutation  may  arise  simultaneously  in 
many  individuals  instead  of  as  a  single  ''  sport." 

/.  Mutations  do  not  vary  around  an  arithmetical 
mean  with  respect  to  the  parent  form,  as  is  the  case 


MUTATION  73 

with  fluctuating  variations,  but  they  fluctuate  around 
a  new  average  of  their  own,  thus  forming  a  discon- 
tinuous series  with  the  parent  form. 

g.  Mutations  may  occur  in  all  directions,  that  is, 
they  are  not  necessarily  definite  or  orthogenetic. 

h.   Mutations  probably  appear  periodically. 

i.  Every  mutation  means  a  possible  doubling  of  the 
species. 

j.  Useless  or  insignificant  fluctuating  variations  are 
not  necessarily  the  material  from  which  natural  selec- 
tion must  sift  out  new  species. 

The  bearing  of  the  whole  matter  of  mutation  upon 
heredity  lies  in  the  fact  that,  contrary  to  Darwin's 
belief,  it  is  apparently  mutations,  and  not  fluctuations, 
that  make  up  heritable  variations.  If  this  supposition 
proves  to  be  true,  mutations  furnish  the  essential 
material  in  the  study  of  heredity.  Consequently, 
whatever  knowledge  we  may  gain  of  them  has  a  direct 
relation  to  the  entire  problem  of  genetics. 


CHAPTER  V 

THE   INHERITANCE    OF   ACQUIRED    CHARACTERS 
1.   Summary  of  Preceding  Chapters 

Hereditary  resemblance  is  due  to  the  derivation  of 
offspring  from  the  same  stock  as  the  parent,  and  suc- 
cessive generations,  therefore,  are  simply  periodic 
expressions  of  the  same  continuous  stream  of  germ- 
plasm. 

Perfect  inheritance,  or  uniformity  of  generations, 
does  not  exist,  since  variations  always  occur  in  suc- 
cessive generations.  It  is  upon  these  variations  that 
evolution  depends.  Without  them  there  would  be 
no  change  of  type  and  consequently  no  possibility 
of  evolutionary  advance. 

Some  variations  are  fluctuating  or  continuous  in 
character  and  may  be  detected  and  analyzed  by 
statistical  methods,  while  others  are  mutations,  or 
discontinuous  variations,  representing  qualitative 
differences  which  do  not  lend  themselves  readily  to 
statistical  analysis. 

Mutations  are  more  common  than  was  formerly 
believed,  and  since  they  are  germinal  rather  than 
somatic  in  character,  they  play  an  important  role 
in  heredity. 

74 


INHERITANCE  OF  ACQUIRED  CHARACTERS  75 

2.   The  Bearing  of  this  Chapter  upon  Genetics 

Only  those  variations  which  reappear  in  succeeding 
generations  have  to  do  with  heredity.  Hence  it  be- 
comes important  to  inquire  as  to  what  kind  of  varia- 
tions actually  reappear.  Can  variations  that  are  not 
inborn,  but  which  are  acquired  during  the  lifetime  of 
the  individual,  be  inherited  ?  Does  the  experience  of 
the  parent  become  a  direct  part  of  the  child's  heritage, 
or  can  the  environment  of  the  one  enter  in  any  way 
into  the  heredity  of  the  other  ?  Can  changes  wrought 
in  the  somatoplasm  be  so  impressed  upon  the  germ- 
plasm  as  to  change  it  in  such  a  way  that  it,  in  turn, 
will  give  rise  to  similarly  modified  somatoplasm  in  the 
next  generation  ?  Can  nurture  as  well  as  nature  be 
transmitted  ? 

In  answering  these  questions  we  are  of  course  con- 
cerned solely  with  biological  inheritance  and  not  at 
all  with  those  extra-biological  accumulations  in  the 
way  of  arts,  literature,  tradition,  invention,  and  the 
like  which  constitute  civilization  and  which  make 
us  the  "heirs  of  the  ages."  Such  benefits  are  entailed 
upon  us  much  in  the  same  way  as  property  is  "in- 
herited," but  they  form  no  part  of  the  personal  bio- 
logical heritage  into  which  we  are  now  inquiring. 

3.  The  Importance  of  the  Question 

This  inquiry  concerning  the  inheritance  of  acquired 
characters,  which  Professor  Brooks  has  called  "the 
interminable  question,"  is  not  simply  an  academic 
matter.      Its   solution   is   of   vital   importance   from 


76  GENETICS 

several  viewpoints.  For  breeders,  who  are  trying  to 
maintain  or  improve  particular  strains  of  animals  or 
plants;  for  physicians,  who,  in  fighting  disease,  are 
honestly  seeking  to  substitute  an  ounce  of  prevention 
for  a  pound  of  cure ;  for  sociologists  and  philanthro- 
pists, who  have  at  heart  the  permanent  bettering  of 
human  conditions ;  for  educators,  who  cherish  hopes 
that  their  life-work  of  unfolding  the  youthful  mind 
may  prove  cumulative  and  lasting  rather  than  tran- 
sitory; for  religious  workers,  who  want  their  faith 
strengthened  that  conquests  in  character-building 
may  outreach  the  individual  and  so  enrich  the  race ; 
for  parents,  who  entertain  hopes  that  their  own  efforts 
may  give  their  children  a  better  biological  start  in 
life,  —  for  all  these  and  many  more,  it  is  important 
to  know  the  answer  to  the  question :  Can  acquired 
characters  be  inherited  ? 

4.   An  Historical  Sketch  of  Opinion 

That  the  personal  accumulations  of  a  lifetime  are 
heritable  was  generally  believed  all  through  the 
credulous  ages.  A  century  ago  Lamarck  made  this 
idea  the  corner-stone  of  his  theory  of  evolution.  It 
was  all  very  simple.  The  reason  evolution  occurs 
in  nature  is  because  individual  acquirements  are 
being  continually  added  to  the  onflowing  stream 
of  living  forms.  This  cumulation  of  characters 
indeed  is  evolution.  How  else  can  the  present  stage 
of  adaptation  of  organisms  to  their  several  niches  in 
nature  be  explained  save  by  seeing  in  it  the  final  results 
of  generations  of  gradually  inherited  adaptations  ? 


INHERITANCE  OF  ACQUIRED  CHARACTERS  77 

Darwin  also  believed  in  the  inheritance  of  acquired 
characters,  although  he  differed  from  Lamarck  with 
respect  to  how  such  characters  are  acquired. 

Francis  Gal  ton  in  1875  was  one  of  the  first  to  ex- 
press skepticism  regarding  this  generally  accepted 
belief  but  the  man  who,  in  a  masterly  manner,  fo- 
cused the  growing  doubt,  and  who  did  more  than 
any  other  to  inspire  thought  and  investigation  upon 
the  subject,  was  August  Weismann,  for  nearly  fifty 
years  professor  in  the  University  of  Freiburg  in 
Baden.  Weismann  made  the  issue  so  clear  that  the 
heritability  of  acquired  characters  became  the  parting 
of  the  ways  which  divided  biologists  into  the  two 
camps  of  Neo- Lamar ckians  who  affirm,  and  Neo- 
Darwinians  who  deny,  such  inheritance.  If  the 
question  could  be  decided  by  a  vote  or  by  an  expres- 
sion of  opinion,  the  result  would  be  doubtful,  since 
each  column  contains  the  names  of  men  whose  scien- 
tific accomplishments  entitle  them  to  a  respectful 
hearing.  Geneticists  and  embryologists,  represent- 
ing the  two  lines  of  study  which  furnish  the  most 
immediate  approach  to  this  problem,  are  well-nigh 
agreed,  however,  that  acquired  characters  are  not 
inherited. 

But  just  what  are  the  facts  of  the  case  ? 

5.  Confusion  in  Definitions 

The  source  of  much  of  the  lack  of  agreement  in 
this  controversy  lies  in  the  definition  of  what  con- 
stitutes an  "acquired  character."  One  is  reminded 
of  the  two  knights  who  fought  so  bitterly  over  the 


78  GENETICS 

color  of  a  shield,  one  maintaining  that  it  was  red, 
the  other  that  it  was  black.  So  they  hacked  away 
at  each  other,  as  all  good  knights  should  do  in  the 
defense  of  the  truth,  until  they  both  fell  down  dead 
beside  the  shield  which  was  black  on  one  side  and 
red  on  the  other. 

Of  course  actual  characters  are  never  inherited,  but 
only  the  determiners  or  potentialities  which  regulate 
the  way  in  which  the  organism  reacts  to  its  environ- 
ment or  training  with  respect  to  the  characters  in 
question.  Reid  has  pointed  out  that  in  one  sense 
every  adult  character  is  "acquired"  because  it  has 
no  expression  at  first.  For  instance,  there  is  no 
beard  on  the  face  of  a  male  infant,  but  it  will  presum- 
ably be  "acquired"  later  on  in  the  life-cycle. 

It  is  plain  that  every  new  character  which  repre- 
sents a  forward  evolutionary  step  must  have  been 
"acquired,"  or  added,  sometime  and  somewhere,  else 
it  would  not  be  present,  as  there  is  evidence  that  it  is. 
Perhaps  the  question,  as  Montgomery  has  suggested, 
ought  to  be  changed  to  read  :  "  What  kinds  of  acquired 
characters  are  inherited.^"  It  is  obvious  that  dis- 
cussion is  futile  until  a  common  denominator  in  the 
shape  of  a  definition  of  acquired  characters  shall  be 
accepted. 

6.   Weismann's  Conception  of  Acquired 

Characters 

Weismann  defines  an  acquired  character  as  any 
somatic  modification  that  does  not  have  its  origin  in  the 
germplasm. 


INHERITANCE  OF  ACQUIRED  CHARACTERS  79 

Of  course  those  somatic  modifications  which  are 
phases  of  the  developing  individual,  such  as  the 
acquisition  of  a  deeper  voice  at  puberty  or  the  sub- 
stitution of  the  permanent  dentition  for  the  milk- 
teeth,  are  somatic  variations  which  have  their  rise 
and  control  in  the  germplasm  and  consequently 
cannot  properly  be  included  under  the  head  of  ac- 
quired characters. 

Examples  of  acquired  characters  in  the  Weis- 
mannian  sense  are  mutilations,  the  effects  of  environ- 
ment, the  results  of  function  as  in  the  use  or  disuse  of 
certain  organs,  and  such  diseases  as  may  be  due  either 
to  invading  bacteria  or  to  the  neglect  or  abuse  of 
the  bodily  mechanism. 


7.   The  Distinction  between  Germinal  and 
Somatic  Characters 

Redfield  has  recently  thrown  light  on  the  classi- 
fication of  the  characters  which  make  up  the  indi- 
vidual by  quoting  the  familiar  lines  :  — 

"  Some  are  horn  great, 
Some  achieve  greatness, 
Some  have  greatness  thrust  upon  them. 


»> 


"Born"  characters  are  constitutional,  having 
their  origin  in  the  germplasm  itself.  They  are 
never  Weismannian  acquired  characters  and  may  be 
illustrated  by  eye-color,  mental  disposition,  or  facial 
features.  Lightning  calculators  and  musical  prodi- 
gies may  have  their  gifts  developed   and  enlarged. 


80  GENETICS 

but  the  fact  that  their  talent  is  nevertheless  an 
unmistakable  gift,  and  not  an  acquisition,  remains 
true. 

"Achieved"  characters  are  functional  and  are 
gained  by  exercise.  Many  things  are  achieved, 
however,  which  are  not  acquired  characters,  as, 
for  instance,  wealth,  reputation,  or  an  education.  Not 
any  of  these  are  biological  characters,  and  therefore 
we  are  not  concerned  with  them  in  this  connection, 
although  in  the  case  of  education  it  should  be  noticed 
that  the  mental  exercise  necessary  to  bring  about  a 
trained  mind,  if  not  the  subject  matter  of  the  edu- 
cation itself,  is  distinctly  an  acquired  character  of 
the  "achieved"  type. 

"Thrust"  characters  are  the  results  of  environ- 
ment. They  are  acquired  w^ithout  functional  activity 
on  the  part  of  the  organism  and  usually  in  spite  of 
anything  the  organism  can  do  to  prevent.  Some- 
times these  characters  are  thrust  upon  the  individual 
before  birth,  as  in  the  case  of  blindness  caused  by 
parental  gonorrhoea  or  tuberculosis  arising  from 
uterine  infection,  in  which  case  they  are  termed  con- 
geiiital  characters. 

Congenital  or  prenatal  characters,  however,  are  in 
no  way  the  same  as  germinal  characters,  for  they  fall 
just  as  truly  into  the  category  of  acquired  variations 
as  do  those  which  make  their  appearance  in  later  life. 

8.   What  Variations  reappear  ? 

Returning  now  to  Montgomery's  question,  —  "WTiat 
kinds    of    acquired    characters    are    inherited.^"  —  it 


INHERITANCE  OF  ACQUIRED  CHARACTERS  81 

is  apparent  that  only  the  "born"  ones  can  be,  which 
have  their  roots  in  the  germplasm  whence  tlie  new 
individual  arises,  and  that  "achievements"  and 
"thrusts,"  in  order  to  reappear  in  the  succeeding  gen- 
eration, can  do  so  only  by  first  becoming  incorporated  in 
the  germplasm. 

Any  character  that  is  not  acquired  must  have  been 
present  in  the  germplasm  from  which  the  organism 
arose,  as  there  is  no  transfer  of  characters  between 
organisms  except  through  the  germ-cells.  Thus  it  is 
evident  that  the  only  inherited  acquisitions  are  those 
which,  either  primarily  or  secondarily,  bring  about 
variation  in  the  germplasm.  Such  temporary  acquisi- 
tions as  a  coat  of  tan  or  a  display  of  freckles  do  not 
impress  the  germplasm,  for  when  the  cause  that  in- 
cites their  appearance  is  removed,  they  soon  vanish. 

9.   What  may  cause  Germplasm  to  vary  or  to 
ACQUIRE  New  Characters  ? 

The  causes  which  bring  about  changes  in  the  germ- 
plasm may  be  either  internal  or  external. 

Of  possible  internal  causes  may  be  mentioned  first 
the  "amphimixis"  of  Weismann,  that  is,  the  mixture 
of  two  nearly  related  strains  of  germplasm  in  sexual 
reproduction  within  a  species,  or  secondly,  the  mixture 
of  two  more  remotely  related  strains  resulting  in  hy- 
bridization. In  either  case  the  strain  of  germplasm 
undergoes  a  shake-up  that  may  result  at  least  in  new 
combinations  of  characters,  if  not  in  the  production 
of   entirely  new  characters.      This  recombination  of 

G 


82  GENETICS 

characters  in  amphimixis  and  hybridization  will  re- 
ceive further  attention  in  a  later  chapter. 

The  fact  that  successive  parthenogenetic  genera- 
tions, in  which  amphimixis  does  not  of  course  occur, 
may  show  a  larger  degree  of  variability  than  sexually 
produced  generations,  indicates  that  amphimixis  in 
itself  is  by  no  means  sufficient  to  account  for  all  kinds 
of  variations. 

The  abrupt  way,  for  instance,  in  which  mutations 
appear  in  apparent  independence  of  external  influences 
suggests  that  there  may  be  some  internal  factor,  as 
yet  unknow^n,  acting  directly  through  the  germplasm, 
regardless  of  external  causes. 

The  assumption  of  an  unknown  factor  does  not 
necessarily  imply  a  return  to  "vitalism,"  which  is  so 
elusive  of  experimental  test  and  hence  so  unsatisfac- 
tory to  the  scientific  mind,  nor  does  it  admit,  simply 
because  this  factor  is  at  present  an  unknown  quantity, 
that  it  is  consequently  doomed  to  remain  so. 

It  is  easily  conceivable  that  the  external  factors 
acting  upon  the  germplasm  may  be  grouped  into  two 
alternative  classes  :  first,  external  factors  that  act  upon 
the  somatoplasm  and  through  the  agency  of  the 
somatoplasm  affect  the  germplasm ;  and  second,  those 
that  act  directly  upon  the  germplasm  without  neces- 
sarily at  the  same  time  influencing  the  somatoplasm. 

The  first  category,  that  of  somatic  modifications 
which  leave  their  impress  upon  the  germplasm,  in- 
cludes true  acquired  characters  according  to  our 
definition,  while  the  second,  which  includes  cases  of 
the    direct    influence    of    external    stimuli    upon    the 


INHERITANCE   OF  ACQUIRED   CHARACTERS   83 

germplasm,  regardless  of  any  simultaneous  modifica- 
tion of  the  somatoplasm,  must  be  excluded  as  irrele- 
vant to  a  discussion  of  the  heritability  of  acquired 
characters  in  the  Weismannian  sense,  since  thev  are 
not  somatic  modifications  at  all. 

Many  instances  of  direct  influence  of  external 
stimuli  upon  germplasm  are  known  in  biological 
literature,  and  these  have  led  to  some  of  the  misunder- 
standings concerning  the  "interminable  question"  of 
the  inheritance  of  acquired  characters. 

MacDougall,  for  example,  was  able  by  injecting 
certain  salts  into  the  carpels  of  plants  to  stimulate 
the  germplasm  of  the  forming  seeds  so  directly  that 
a  progeny  of  modified  character  was  produced  which, 
in  succeeding  generations,  bred  true  to  the  newly  in- 
duced character. 

Sitkowski,  also,  fed  the  caterpillars  of  the  moth 
Tineola  bisellieUa  with,  an  aniline  dye  (Sudan  red  III), 
obtaining  therefrom,  instead  of  the  normal  whitish 
ones,  moths  that  laid  colored  eggs,  and  these  in  turn 
hatched  into  caterpillars  still  tinged  with  the  color  of 
the  red  dye.  Riddle,  with  guinea-pigs,  and  Gage, 
with  poultry,  obtained  quite  similar  results.  This  is 
an  instance  of  what  has  been  termed  *' parallel  induc- 
tion "  where  somatoplasm  and  germplasm  are  affected 
together  by  an  external  factor,  as  opposed  to  "  somatic 
induction "  or  Weismannian  acquired  characters  in 
which  the  germplasm  is  secondarily  influenced  through, 
or  by  the  agency  of,  the  somatoplasm. 


84  GENETICS 

10.  Weismann's  Reasons  for  doubting  the  Inher- 

itance OF  Acquired  Characters 

Weismann's  reasons  for  questioning  the  popularly 
accepted  view  that  acquired  characters  are  inherited 
may  be  briefly  stated  as  follows :  — 

First,  there  is  no  known  mechanism  whereby 
somatic  characters  may  be  transferred  to  the  germ- 
cells. 

Second,  the  evidence  that  such  a  transfer  actually 
does  occur  is  inconclusive  and  unsatisfactory. 

Third,  the  theory  of  the  continuity  of  the  germ- 
plasm  is  sufficient  to  account  for  the  facts  of  heredity 
without  assuming  the  inheritance  of  acquired  somatic 
characters. 

Let  us  examine  these  three  statements  a  little  more 
closely. 

11.  No  Known   Mechanism  for  impressing  the 

Germplasm  with  Somatic  Characters 

The  somatoplasm  is  something  that  has  traveled 
out  from  the  original  fundamental  germplasm  along 
the  paths  of  differentiation  and  elaboration.  The 
more  complex  the  body  cells  become,  that  is,  the 
more  successive  modifications  they  undergo,  the  more 
difficult  it  is  for  these  somatic  cells  to  return  to  their 
original  primitive  estate. 

In  many  lower  forms  of  life  where  cell  elaboration 
is  not  so  great,  a  part  lost  by  amputation  is  often 
regenerated,  but  this  process  is  not  possible  in  higher 


INHERITANCE  OF  ACQUIRED  CHARACTERS  85 

forms  where  the  parts  represent  cell  complexes  too 
hopelessly  differentiated  to  begin  anew  the  unfolding 
sequences  of  their  elaboration.  This  difficulty  was 
a  very  real  one  in  the  mind  of  that  famous  nocturnal 
inquirer  Nicodemus  when  he  asked:  "How  can  a  man 
be  born  when  he  is  old  ?  Can  he  enter  a  second  time 
into  his  mother's  womb  and  be  born  ?" 

Not  only  the  development  of  the  race  which  we 
call  evolution,  but  also  the  determination  of  the 
individual  in  heredity,  is  a  chain  of  omvard-moving 
sequences  like  the  succession  of  events  in  history.  It 
is  hard  to  see  how  recent  events  can  influence  pre- 
ceding events.  It  is  hard  to  see  how  the  water  that 
has  gone  over  the  dam  can  return  and  affect  the  flow 
of  the  river  upstream  in  any  direct  way.  It  is  like- 
wise hard  to  see  how  differentiated  somatoplasm,  which 
represents  the  end  stage  of  a  successive  series  of 
modifications,  can  make  any  definite  impress  upon 
the  original  germplasmal  sources  from  which  it  arose. 

Darwin  felt  this  difficulty  and  presented  with  apolo- 
gies his  provisional  hypothesis  of  pangenesis  in  which 
he  assumed  that  every  bodily  part  sends  contributions 
to  the  germ-cells  in  the  form  of  "gemmules."  These 
gemmules,  or  hypothetical  somatic  delegates,  then 
reconstruct  in  the  germ-cells  the  characters  of  the  en- 
tire body,  including  acquired  modifications  as  well  as 
all  others,  and  thus  there  is  no  reason  why  acquired 
characters  cannot  readily  be  transmitted.  Unfortu- 
nately there  is  no  tangible  basis  in  fact  for  this  de- 
lightfully simple  explanation  to  rest  upon.  It  is  a 
theory  assuming  that  all  parental  somatic  cells  take 


86  GENETICS 

part  in  the  formation  of  the  new  individual,  hence 
it  was  called  *' pangenesis,"  or  origin  from  all. 

Nothing  we  have  subsequently  learned  of  minute 
cell  structure  favors  this  hypothesis,  while  many 
facts  go  quite  against  it.  Moreover,  it  is  directly 
opposed  to  the  theory  of  the  continuity  of  germplasm 
so  convincingly  set  forth  later  on  by  Weismann. 
Darwin  indeed  advanced  it  only  in  the  most  tenta- 
tive way,  being  entirely  ready  to  see  it  abandoned  at 
any  time  for  something  better.  It  at  least  per- 
formed one  valuable  service  to  science,  namely,  that 
of  demonstrating  how  far  investigators  were  from 
an  adequate  conception  of  any  means  by  which 
somatic  modifications  might  become  incorporated  in 
the  germ-cells. 

We  must  acknowledge,  however,  with  Lloyd 
Morgan  that  the  fact  that  a  mechanism  for  the  trans- 
fer of  somatic  characters  to  the  germ-cells  has  not 
been  discovered,  is  not  proof  that  such  a  mechanism 
does  not  exist.  It  may  simply  be  beyond  our  present 
powers  of  penetration. 

12.   Evidence  for  Transmission  of  Acquired 
Characters  Inconclusive 

The  evidence  for  the  inheritance  of  acquired 
characters  was,  for  a  long  time,  taken  for  granted. 
This  theory  was  the  most  obvious  explanation  of 
many  facts  and  so  was  accepted  without  question. 
An  obvious  interpretation,  however,  is  not  always  the 
correct  one.  The  sun  appears  to  go  around  the 
earth,  but  astronomers  assure  us  that  it  does  not. 


INHERITANCE  OF  ACQUIRED  CHARACTERS  87 

When  Weismann  began  to  sift  the  evidence  for 
the  inheritance  of  acquired  characters,  he  found  tliat 
it  was  largely  based  upon  opinion  rather  than  fact, 
much  like  the  popular  belief  with  regard  to  prenatal 
influences  and  birthmarks,  or  the  causation  of  warts 
by  handling  toads. 

The  supposed  evidence  for  the  inheritance  of  ac- 
quired characters  falls  chiefly  into  four  categories  :  — 

a.  Mutilations ; 

h.  Environmental  effects; 

c.  The  effects  of  use  or  disuse ; 

d.  The  transmission  of  disease. 

a.  Mutilations 

It  is  fortunate  that  the  sons  of  warriors  do  not 
inherit  their  fathers'  honorable  scars  of  battle,  else 
we  would  now  be  a  race  of  cripples. 

The  feet  of  Chinese  women  of  certain  classes  have 
for  centuries  been  mutilated  into  deformity  by  band- 
aging, without  the  mutilation  in  any  way  becoming 
an  inherited  character.  The  same  result  is  also 
true  of  circumcision,  a  mutilation  practised  from 
ancient  times  by  the  Jews  and  certain  other  Eastern 
peoples.  The  progressive  degeneration  or  crippling 
of  the  little  toe  in  man  has  been  explained  as  the 
inheritance  of  the  cramping  effect  of  shoes  upon 
generations  of  shoe  wearers,  but,  as  Wiedersheim 
has  pointed  out,  the  fact  that  Egyptian  mummies 
show  the  same  crippling  of  the  little  toe  is  unfavorable 
to  this  hypothesis,  for  no   ancient  Egyptian  could 


88  GENETICS 

ever  be  accused  of  wearing  shoes  or  of  having  had 
shoe- wearing  ancestors. 

Sheep  and  horses  with  docked  tails  as  well  as  dogs 
with  trimmed  ears  never  produce  young  having  the 
parental  deformity.  Weismann's  classic  experiment 
with  mice,  an  experiment  subsequently  confirmed 
by  others,  is  additional  negative  evidence  upon  this 
same  point. 

What  Weismann  did  was  to  breed  mice  whose 
tails  had  been  cut  off  short  at  birth.  He  continued 
this  decaudalization  through  twenty-two  generations 
with  absolutely  no  effect  upon  the  tail-length  of  the 
new-born  mice.  One  may  see  in  the  catacombs  of 
the  Zoologisches  Institut  at  Freiburg,  filed  carefully 
away  on  shelves,  as  a  "  document,"  long  rows  of  labeled 
bottles  containing  the  fifteen  hundred  and  ninety-two 
martyrs  to  science  which  made  up  the  twenty-two 
generations  of  mice  in  this  famous  experiment. 

Conklin  has  hit  the  nail  upon  the  head  with  respect 
to  mutilations  by  saying:  "Wooden  legs  are  not  in- 
herited, but  wooden  heads  are." 

b.  Environmental  Effects 

Trees  deformed  by  prevailing  winds,  like  the 
willows  that  line  the  canals  in  Belgium  and  Holland, 
or  storm-crippled  trees  along  the  exposed  seacoast 
are  not  known  to  produce  a  modified  progeny 
when  their  adverse  environmental  conditions  are 
removed.  Similarly,  the  persistent  sunburn  of  Eng- 
lishmen long  resident  in  India  does  not  reappear  in 
their  children  born  in  England. 


INHERITANCE  OF  ACQUIRED  CHARACTERS  89 

Sumner  kept  mice  in  a  constant  but  abnormally 
high  temperature  of  26°  C.  with  the  result  that  the 
ears,  tail,  and  feet  grew  noticeably  larger  than  in 
control  animals  kept  in  ordinary  lower  temperatures, 
while  at  the  same  time  the  general  hairiness  of  the 
body  decreased.  It  should  be  remembered,  however, 
that  mice  are  mammals  which  pass  through  an  ex- 
tended uterine  existence,  so  that  it  is  easy  to  see  how 
the  offspring  in  this  case  were  subjected  to  the  same 
excessive  temperature  as  the  parents  for  a  period 
sufficient  to  amply  account  for  their  subsequent 
variation  when  removed  to  a  normal  environ- 
ment. 

Lederbaur  finds  that  the  wayside  weed  Capsella, 
which  in  the  course  of  many  years  has  gradually 
crept  along  the  roadside  up  into  an  Alpine  habitat 
and  there  "acquired"  Alpine  characters,  upon  being 
transplanted  to  the  lowlands  retains  its  Alpine 
modifications.  Although  this  case  has  been  cited 
as  an  authentic  instance  of  the  inheritance  of  ac- 
quired characters,  is  it  not  possible  that  the  conquest 
of  the  Alps  by  Capsella  has  been  due,  in  the  course 
of  time,  not  to  the  inheritance  of  acquired  characters 
at  all,  but  to  a  gradual  natural  selection  of  just  those 
germinal  variations  which  best  fitted  it  to  cope  with 
Alpine  conditions  until,  finally,  a  strain  of  germplasm 
producing  somatoplasm  suitable  to  Alpine  conditions 
has  been  isolated  in  the  form  of  an  elementary  species 
derived  from  the  original  type  ?  If  this  is  what  has 
happened,  of  course  such  germplasm  would  give 
rise  to  Alpine  plants  whether  individual  plants  grew 


90  GENETICS 

to  maturity  near  the  snow-line  or  in  the  warm  valleys 
at  a  lower  altitude. 

Marie  von  Chauvin  was  able,  by  decreasing  the 
amount  of  water  in  an  aquarium,  to  transform  the 
gill-breathing  salamander  Axolotl  into  the  land  form, 
Ainhly stoma,  which  in  its  adult  form  has  no  gills,  but 
breathes  by  means  of  lungs.  Both  of  these  forms 
are  sexually  mature,  reproducing  their  like,  and  had 
long  been  recognized  by  systematists  as  distinct 
species. 

More  recently  Kammerer,  by  similarly  reducing 
the  water  supply,  succeeded  in  transforming  Sala- 
mandra  maculosa,  a  salamander  that  normally  pro- 
duces about  seventy  eggs  which,  when  hatched  in 
water,  become  gill-breathing  tadpoles,  into  a  sala- 
mander producing  only  two  to  seven  young  which 
are  born  alive  without  gills  and  are  able  to  live  out 
of  water  entirely,  in  damp  situations.  These  land- 
adapted  offspring,  moreover,  when  supplied  with 
abundant  water,  produce  in  turn  tadpoles  which  spend 
days  only,  instead  of  months,  in  the  water  under- 
going their  metamorphosis,  thus  showing  an  appar- 
ent inheritance  of  an  acquired  character. 

It  should  be  pointed  out,  however,  that  in  these 
cases  the  gill-breathing  forms  in  each  instance  rep- 
resent a  case  of  arrested  development.  Axolotl  is 
simply  a  larval  form  of  Amhlystoma  that,  under  nor- 
mal conditions  of  an  abundant  water  environment 
and  high  temperature,  gets  no  further  in  its  meta- 
morphosis than  the  tadpole  stage,  when  it  produces 
eggs  and  sperms  and  finishes  its  life  story.     A  change 


INHERITANCE  OF  ACQUIRED  CHARACTERS  91 

in  environment  simply  permits  the  life-cycle  to  go 
on  further.  Changing  from  gill-breathing  to  lung- 
breathing  is  not,  therefore,  an  acquired  character, 
but  a  purely  germinal  character  that  may  be  either 
blocked  or  released  by  changing  conditions  in  the 
environment. 

c.   The  Effects  of  Use  or  Disuse 

The  callosities  on  the  end  of  a  violinist's  left-hand 
fingers  are  acquired  by  use,  but  they  are  not  inherited. 
There  are  callosities  on  the  knees  of  the  wart-hog, 
Phacechcerus,  which  are  also  apparently  the  result 
of  use,  for  these  animals  kneel  as  they  root  for  a  living 
in  the  African  forests,  and  have  done  so  for  untold 
generations.  It  has  been  noticed  that  young  wart- 
hogs  as  soon  as  they  are  born  possess  the  callosities, 
so  that  this  instance  looks  like  one  of  inheritance  of 
a  character  acquired  through  use  or  exercise. 

The  skin  on  the  soles  of  human  feet  is  thicker  than 
the  skin  elsewhere,  and  by  use  it  becomes  still  thicker. 
This  is  apparently  another  instance  of  the  same  sort. 
The  writer  has  observed,  however,  that  a  cross  sec- 
tion through  the  foot  of  a  "mud  puppy,"  Necturus 
maculatus,  shows  a  much  thickened  sole.  Necturus, 
it  should  be  noted,  is  a  very  primitive  salamander 
living  always  under  water  and  never  using  the  soles 
of  its  feet  in  any  way  to  bear  its  weight,  nor  is  it 
reasonable  to  suppose  that  it  ever  had  any  ancestors 
who  did  so,  for  the  hands  and  feet  of  the  Amphibia 
are  the  most  primitive  and  ancient  hands  and  feet  to 
be  found  in  the  animal  kingdom  without  any  known 


92  GENETICS 

ancestral  types.  The  thickening  of  the  skin  on  the 
sole  of  the  mud  puppy's  feet  must  be  due,  therefore, 
to  germinal  determiners  and  is  in  no  way  an  acquisi- 
tion through  use.  The  same  may  also  be  true  of  the 
wart-hog's  knees  and  of  human  soles. 

The  strong  arm,  the  skilled  hand,  and  the  trained 
ear  are  not  inherited.  They  have  always  to  be 
reacquired  in  each  succeeding  generation  just  as 
surely  as  the  ability  to  walk,  or  to  read  and  write. 

Herbert  Spencer  has  defined  instinct  as  "inherited 
habit."  But  surely  those  instincts  which  determine 
a  single  isolated  action  during  the  lifetime  of  the 
individual,  such  as  the  spinning  of  a  peculiar  cocoon, 
cannot  be  the  result  of  habit,  since  habits  are  formed 
only  through  repeated  action.  If,  then,  some  in- 
stincts require  a  different  explanation  from  that  of 
"inherited  habit,"  may  it  not  be  likely  that  all  in- 
stincts do  ?  Dr.  Hodge,  who  succeeded  in  hatching 
tame  quail  chicks  out  of  "wild"  eggs,  asks  the  perti- 
nent question:  "How  can  Si  fear  hatch  out  of  an 
eggV  The  habit  of  wildness,  particularly  with 
precocial  chicks  like  quails,  may,  under  an  inciting 
environment,  be  very  soon  established,  but  it  is  diffi- 
cult to  see  how  caution,  gained  by  the  experience  of 
the  parents,  can  find  its  way  into  the  fertilized  egg. 

d.  Disease  Transmission 

Many  diseases,  like  tuberculosis,  have  their  im- 
mediate cause  in  invading  pathogenic  bacteria. 
Bacteria  themselves  cannot  be  inherited  for  the 
reason  that  it  is  not  possible  for  them  to  become  an 


INHERITANCE  OF  ACQUIRED  CHARACTERS  93 

integral  part  of  the  fertilized  egg  and  thus  cross  the 
"hereditary  bridge"  which  joins  two  generations. 
A  general  predisposition  to  bacterial  disease,  that  is,  a 
lack  of  resistance  to  bacterial  invasion  due  to  de- 
fectiveness in  physical  or  physiological  equipment, 
may  be  present  as  a  combination  of  characters  in 
the  germplasm,  or  an  individual,  as  the  result  of 
disease,  may  "acquire"  a  generally  weakened  germ- 
plasm  and  so  produce  a  progeny  exhibiting  general 
liability  to  disease ;  but  it  is  doubtful  if  such  a  con- 
dition can  properly  be  termed  the  inheritance  of 
an  acquired  character,  since  the  particular  definite 
disease  in  question  is  not  demonstrably  heritable. 

When  alcoholism  "runs  in  a  family,"  its  reappear- 
ance in  the  son  is  probably  due  to  the  fact  that  he 
is  derived  from  the  same  weak  strain  of  germplasm 
as  his  father.  The  fact  that  the  father  succumbed 
to  the  alcohol  habit  is  not  the  determining  cause  of 
drunkenness  in  the  son.  The  same  thing  that  caused 
the  father  to  become  an  alcoholic,  namely,  weak 
germplasm,  and  not  the  resulting  drunkenness  in  the 
parent,  is  the  causal  factor  for  alcoholism  in  the  son. 

At  the  same  time  it  is  entirely  probable  that  hered- 
itary alcoholism  may  in  some  cases  arise  through 
"parallel  induction,"  that  is  to  say,  acquired  alco- 
holism may  end  in  the  simultaneous  poisoning  and 
consequent  modification  of  both  the  somatoplasm  and 
germplasm  of  the  parent,  with  the  result  that  the 
germplasm  has  less  resistance  to  alcoholism  in  a  suc- 
ceeding generation.  The  offspring  are  consequently 
more  likely  to  succumb  to  the  disease.     This,  how- 


94  GENETICS 

ever,  is  not  the  inheritance  of  an  acquired  character 
or  of  a  definite  somatic  modification. 

When  a  man  of  the  present  generation  has  rheu- 
matic gout,  it  is  a  severe  stretch  both  of  patriotism 
and  of  the  powers  of  heredity  to  trace  the  origin  of 
the  affliction  back  to  a  revolutionary  ancestor  who 
acquired  sciatic  rheumatism  by  sleeping  on  the 
ground  at  Valley  Forge,  yet  this  is  quite  as  direct  as 
many  alleged  instances  of  the  inheritance  of  disease. 

In  the  majority  of  instances,  apparent  cases  of 
disease  inheritance  are  merely  instances  of  reinfec- 
tion. This  reinfection  of  the  offspring  may  occur 
very  early  in  embryonic  life,  or  it  may  happen  after 
birth,  provided  the  offspring  are  exposed  to  the  same 
environment  as  that  in  which  the  parent  acquired  the 
disease,  but  in  any  case  reinfection  is  not  heredity , 

13.   The  Germplasm  Theory   sufficient   to  ac- 
count FOR  the  Facts  of  Heredity 

Weismann  holds  that  the  theory  of  the  continuity 
of  the  germplasm,  already  considered  in  a  previous 
chapter,  is  sufficient  in  itself  to  account  for  the  facts  of 
heredity.  Hence  it  is  quite  unnecessary  to  fall  back 
upon  the  inheritance  of  acquired  characters  as  an  ex- 
planation, since  this  theory  is  at  least  difficult,  if  not 
impossible,  of  satisfactory  proof. 

To  prove  the  inheritance  of  acquired  characters, 
according  to  Weismann  three  things  are  necessary : 
firsts  a  particular  somatic  character  must  be  called 
forth  by  a  known  external  cause ;  second,  it  must  be 
something  new  or  different  from  what  was  already 


INHERITANCE  OF  ACQUIRED  CHARACTERS  95 

exhibited  before,  and  not  be  simply  the  reawakening 
of  a  latent  germinal  character;  and  third,  the  same 
particular  character  must  reappear  in  succeeding 
generations  in  the  absence  of  the  original  external 
cause  which  brought  the  character  in  question  forth. 
As  yet  these  conditions  have  not  been  convincingly 
met  in  the  evidence  which  has  been  brought  forward 
in  support  of  the  inheritance  of  acquired  characters. 

14.   The  Opposition  to  Weismann 

The  opponents  of  Weismann  point  out,  as  a  weak 
place  in  his  argument,  the  assumption  that  the  germ- 
plasm  is  so  insulated  from  the  somatoplasm  as  not  to 
be  influenced  by  it.  Weismann  assumes,  of  course, 
that  the  germplasm  is  isolated  from  the  somatoplasm 
very  early  in  the  development  of  the  fertilized  egg 
into  an  individual,  and  that  when  once  isolated  it  there- 
after takes  no  active  part  in,  nor  is  in  any  way  affected 
by,  the  vicissitudes  through  which  the  somatoplasm, 
or  the  body  itself,  passes.  The  somatoplasm  is  thus 
merely  a  carrier  of  the  germplasm  and  unable  to 
affect  the  character  of  it  any  more  than  a  rubber  hot- 
water  bag,  although  capable  of  assuming  a  variety 
of  shapes,  can  affect  the  character  of  the  water  within 
it. 

In  opposition  to  this  view  it  is  urged  that  every 
organism  is  a  physiological  as  well  as  a  morphological 
unity,  and  that  cells  entirely  insulated  within  such  a 
unity  would  be  a  physiological  miracle. 

There  is  abundant  evidence  that  germ-cells,  or 
rather  the  sexual  organs  producing  the  germ-cells,  do 


96  GENETICS 

affect  the  somatoplasm  under  particular  conditions, 
as,  for  example,  in  cases  of  castration  when  those 
somatic  features  called  "secondary  sexual  charac- 
ters" undergo  profound  modification. 

If  the  germplasm  thus  exercises  a  constant  influ- 
ence on  the  somatoplasm,  why,  it  seems  legitimate  to 
ask,  may  not  the  reverse  be  true  and  acquired  somatic 
characters  leave  their  impress  upon  the  germ-cells  ? 

15.  Conclusion 

But  even  granting  the  reverse  to  be  true,  that  is, 
that  the  somatoplasm  affects  the  germ-cells,  the  in- 
heritance of  acquired  characters  is  by  no  means 
thereby  established. 

In  order  to  do  this,  the  precise  acquired  character 
in  question,  which  indirectly  exercised  its  influence 
upon  the  germ,  must  be  redeveloped,  and,  although 
the  germplasm  might  conceivably  receive  an  influ- 
ence from  the  somatoplasm  and  be  affected  by  it  in 
a  general  way,  it  is  a  different  matter  entirely  to 
develop  anew  the  verisimilitude  of  the  character  itself 
which  is  supposed  to  have  been  acquired. 

It  will  be  seen  in  subsequent  pages,  under  the  dis- 
cussion of  data  furnished  by  experimental  breeding, 
that  the  weight  of  probability  is  decidedly  against 
the  time-honored  belief  in  the  inheritance  of  acquired 
characters. 


CHAPTER  VI 

THE    PURE    LINE 
1.   The  Unit  Character  Method  of  Attack 

In  reducing  any  body  of  facts  to  a  science,  it  is 
first  necessary  to  determine  the  underlying  units 
out  of  which  the  facts  are  made  up. 

Chemistry  was  alchemy  until  the  chemical  ele- 
ments were  identified  and  isolated.  Histology  was 
terra  ohscura  until  the  cell  theory  brought  forward 
"cells"  as  the  units  of  tissues.  In  the  same  way 
there  could  be  no  science  of  genetics  until  the  con- 
ception was  developed  that  the  individual  is  a  bundle 
of  unit  characters  rather  than  a  unit  in  itself.  So  it 
has  come  about  that  we  now  speak  of  inheritance  as 
applied  to  unit  characters  rather  than  to  individuals 
as  a  whole. 

Incidentally  the  fact  that  an  organism  is  a  com- 
bination of  many  units  makes  it  easy  to  account  for 
the  wide  diversity  of  forms  found  in  nature,  since  the 
addition  of  a  single  unit  greatly  increases  the  pos- 
sible combinations  in  successive  generations. 

Thus  if  three  unit  characters.  A,  B,   and   C,  are 

present  in  each  parent,  for  example,  there  would  be 

six  possible   double   combinations   in   the  offspring, 

namely,  AA,  AB,  AC,  BB.  BC,  and  CC.     If  now  a 

H  97 


98  GENETICS 

fourth  unit  D  is  added  by  one  parent,  there  would  be 
not  only  the  original  six  double  combinations,  but  in 
addition  to  these,  AD,  BD,  and  CD,  that  is,  as  many 
more  as  there  are  unit  characters  with  which  the 
new    one    may    combine. 

Obviously,  when  individuals  are  made  up  of  very 
many  unit  characters,  as,  for  instance,  a  thousand,  the 
addition  of  one  new  unit  character  will  increase  the 
possible  double  combinations  a  thousand  fold. 

2.   Galton's  Law  of  Regression 

Galton  was  one  of  the  first  ^  to  attempt  to  express 
mathematically  the  relationship  between  parents 
and  offspring  by  means  of  treating  statistically  a 
single  unit  character.  According  to  Galton,  a  mathe- 
matical expression  of  the  relationship  between  two 
generations  should  serve  as  a  corner-stone  of  heredity. 

What  Galton  did  was  to  take  human  stature  as  a 
unit  character  in  comparing  204  English  parents  and 
their  928  adult  offspring,  because  human  stature  is 
not  complicated  by  environmental  influences  and  is, 
consequently,  a  purely  hereditary  matter. 

Since  female  height  is  normally  less  than  male 
height,  the  two  were  reduced,  for  purposes  of 
comparison,  to  a  common  male  denominator  by 
multiplying  each  female  height  by  1.08,  which  is 
the  average  amount  that  the  male  exceeds  the 
female  in  height.  There  are  always  two  parents  con- 
cerned in  the  sexual  production  of  every  offspring. 


1 " 


Hereditary  Genius,"  1869. 


THE   PURE   LINE 


99 


therefore  Gallon  reckoned  a  "midparent"  in  each 
case,  according  to  the  formula 

1.00   $   +  1.08    $ 

in  order  to  represent  the  double  parental  generation 
by  a  single  number  for  the  purpose  of  easy  compari- 
son with  the  filial  generation. 


Inches 


Fig.  33.  —  Scheme  to  illustrate  Gallon's  law  of  regression.  The  circles 
represent  graded  groups  of  parental  height  while  the  arrowpoints  in- 
dicate the  average  heights  attained  by  the  respective  offspring.  The 
offspring  of  undersized  parents  are  taller,  and  of  oversized  parents  are 
shorter  than  their  respective  parents.     Based  on  data  from  Galton. 

The    results    of    his    measurements    expressed    in 
inches  are  shown  in  the  following  table,  in  which  the 


100 


GENETICS 


offspring  are,  in  each  instance,  arranged  under  their 
respective  midparents. 


Midparental 

height  .     . 

64.5 

65.5 

66.5 

67.5 

68.5 

69.5 

70.5 

71.5 

72.5 

Average 

height      of 

offspring    . 

65.8 

66.7 

67.2 

67.6 

68.3 

68.9 

69.5 

69.9 

72.2 

The  mean  group  for  all  the  midparents,  it  will  be 
seen,  is  68.5,  and  the  offspring  of  this  group  average 
68.3.  The  table  is  expressed  graphically  in  Figure 
33  in  which  the  circles  connected  by  the  diagonal 
line  represent  the  graded  parental  heights,  while  the 
arrowpoints  indicate  the  average  heights  of  the  off- 
spring in  each  group.  In  order  to  compare  these 
two  series  of  numbers  more  readily,  they  may  be 
reduced  to  a  common  basis  in  which  the  mean  class 
in  each  instance  is  made  equal  to  100,  as  follows :  — 


Midparental 

height  .     . 

94 

95.5 

97 

98.5 

100 

101.5 

103 

104.5 

106 

Average 

height      of 

offspring    . 

96 

97.5 

98.5 

99 

100 

101 

101.5 

102 

105.5 

The  same  series  may  be  expressed  in  terms  of 
amount  of  deviation  from  the  mean  or  middle  classes, 
as  shown  below.  The  deviation  of  each  group  in  the 
series  is  marked  by  the  signs  +  or  —  according  as 
the  heights  given  are  greater  or  less  than  100. 


THE   PURE   LINE 


101 


Midparental 

height,  .     . 

-  6 

-4.5 

-3 

-1.5 

0 

+  1.5 

+  3 

+  4.5 

+  6 

Average 

height      of 

offspring    . 

-  4 

-2.5 

-1.5 

-1 

0 

+  1 

+  1.5 

+  2 

+  5.5 

Finally,  the  relation  between  the  midparent  and 
the  average  offspring  may  be  expressed  in  fractional 
form  by  taking  the  average  height  of  the  offspring 
for  the  numerato]^  and  the  height  of  the  midparent 
for  the  denominator  in  each  instance.  The  minus 
deviations  are  thus  seen  to  be 

6      4.5        2        1.5 

which,  added  together  and  reduced  to  a  decimal, 
equal  .60. 

Similarly,  the  plus  deviations  are 

L5      ~3~      4^      T 

which  reduce  to  .63.  The  average  of  the  minus 
deviations  (.60)  and  the  plus  deviations  (.63)  is 
nearly  .62,  or  about  two  thirds. 

That  is  to  say,  the  fraction  f  represents  the 
amount  of  resemblance  or  "inheritance"  between  two 
generations,  as  determined  by  the  foregoing  series, 
while  the  remaining  |  is  the  measure  of  "regression" 
from  the  general  type. 

This  illustrates  Galton's  Law  of  Regression  or  the 
tendency  in  successive  generations  toward  medioc- 
rity.    The  law  may  be  stated  as  follows  :  — 


102  GENETICS 

Average  parents  tend  to  produce  average  children ; 
minus  parents  tend  to  produce  minus  children ;  plus 
parents  tend  to  produce  plus  children ;  but  the  progeny 
of  extreme  parents,  whether  plus  or  minus,  inherit 
the  parental  peculiarities  in  a  less  marked  degree  than 
the  latter  were  manifested  in  the  parents  themselves. 

3.   The  Idea  of  the  Pure  Line 

It  was  Galton's  law  of  regression  that  suggested 
to  the  Danish  botanist  Johanssen  a  possible  means  of 
controlling  heredity.  In  his  mind  arose  the  ques- 
tion whether  it  would  not  be  possible  by  continually 
breeding  from  plus  parents,  granting  that  plus  par- 
ents produce  plus  offspring  and  making  allowance 
for  some  regression  to  type,  to  shove  over  the  off- 
spring more  and  more  into  the  plus  territory  and  so 
to  establish  a  plus  race. 

To  test  this  hypothesis,  Johanssen  selected  beans, 
Phaseolus,  with  which  to  experiment,  since  this 
group  of  plants  is  self-fertilizing,  prolific,  and  easily 
measurable.  Somewhat  to  his  surprise,  his  beans 
refused  to  shove  over  as  much  as  expected.  That 
is,  big  beans  did  not  yield  principally  big  offspring, 
nor  little  beans  little  offspring,  according  to  the  ex- 
pectation, although  they  each  produced  offspring 
that  varied  in  the  manner  of  fluctuating  variability 
around  an  average  unlike  the  parental  type.  This 
gave  Johanssen  the  idea  that  he  was  using  mixed 
material,  so  he  next  isolated  the  progeny  of  single 
beans,  which,  being  self-fertilized,  each  constituted 
unmistakably  a  single  hereditary  line.     In  this  way 


THE   PURE   LINE  103 

nineteen  beans,  now  famous,  became  the  known 
ancestors  of  Johanssen's  original  nineteen  "pure 
lines,"  a  further  study  of  w^hich  has  led  the  way  to 
some  of  the  most  brilliant  biological  discoveries  of 
recent  years. 

A  pure  line  has  been  defined  by  Johanssen  as  "the 
descendants  from  a  single  homozygous  organism 
exclusively  propagating  by  self-fertilization,"  and 
more  briefly  by  Jennings  as  "all  the  progeny  of  a 
single  self -fertilized  individual." 

4.   Johanssen's  Nineteen  Beans 

It  was  found  by  Johanssen  that  the  progeny  of 
each  of  these  pure  lines  of  beans  varied  around  its 
own  mean,  which  was  different  in  each  of  the  nine- 
teen instances.  When,  however,  extremes  from  any 
pure  line  series  were  selected  and  bred  from,  the  prog- 
eny, instead  of  showing  two  thirds  inheritance  and 
one  third  regression  with  respect  to  the  extremeness 
of  a  particular  character,  as  Galton  found  was  true  in 
the  case  of  human  stature,  showed  no  inheritance  and 
complete  regression  away  from  the  extreme  condition 
of  the  parent  bean  back  to  the  type  for  the  entire 
pure  line  in  question.  That  is,  selection  icithin  a 
pure  line  is  absolutely  without  effect  in  modifying  a 
particular  character  in  the  offspring  of  the  line  in 
question. 

This  is  illustrated  in  Figure  34  in  which  the  results 
of  selecting  for  size  in  the  year  1902  is  shown  for  four 
pure  lines  only.  The  average  for  each  pure  line  is 
given  at  the  top  of  its  column.     When,  for  example, 


104 


GENETICS 


beans  weighing  60  eg.  were  selected  from  pure  lines  IT, 
VII,  and  XV,  the  average  weights  of  their  progenj^ 
were  56.5,  48.2,  and  45.0  eg.  respectively,  which  in 
each  instance  is  nearer  to  the  average  for  the  pure 
line  than  to  the  weight  of  the  parental  seed. 


Weight    of 
Cent,-   parent  «eed 


10    20  30  40    »  60  70       10    20   30  10   50   60  70      10    20   SO  40   50  60  70       10    20  }0    40    SO   60  70       ;0    20    50   40    SO   SO  70 


Pure  line  number 


n 


w 


w 


M 


Fig.  34.  —  The  result  of  selection  in  four  pure  lines  of  beans.  The  verti- 
cal columns,  representing  the  average  progeny  from  different  sized 
parents  all  derived  from  the  same  pure  line,  contain  groups  nearer 
aUke  than  the  horizontal  columns,  representing  progeny  from  the 
same  sized  parents,  but  different  pure  lines.  All  the  numbers  indicate 
centigrams.     Data  from  Johanssen. 

It  will  be  seen  at  once  that  the  averages  in  the 
vertical  columns  are  nearer  alike  than  the  averages 
in  the  horizontal  columns.  In  other  words,  the  beans 
bred  true  to  their  pure  line  rather  than  to  their 
fluctuating  parent. 

As  a  further  example  of  this  law,  take  the  result 


THE   PURE   LINE 


105 


of  selection  for  six  years  in  pure  line  I  as  shown  in 
the  accompanying  table  and  in  Figure  35. 


TTa'rvitqt'  Vitat? 

Mean  Weight  of  Selected 
Parent  Seed 

Mean  Weight  of  Offspring 

Minxis 

Plus 

From  Minus 
Parent 

From  Plus 
Parent 

1902  .... 

1903  .     .     .     . 

1904  .... 

1905  .... 

1906  .... 

1907  .... 

60 
55 
50 
43 
46 
56 

70 
80 
87 
73 
84 
81 

63.15. 

75.19 

54.59 

63.55 

74.38 

69.07 

64.85 
70.88 
56.68 
63.64 
73.00 
67.66 

It  is  evident,  for  instance,  that  in  1907  the  smallest 
beans,  weighing  an  average  of  56  eg.,  gave  an  average 
progeny  weighing  69.07  eg,  while  the  largest  ones 
for  the  same  year,  weighing  an  average  of  81  eg., 
produced  nearly  the  same  average  in  their  progeny 
as  did  the  smallest  beans,  that  is,  67.66  eg. 

Incidentally  all  the  progeny  from  both  large  and 
small  parents  averaged  notably  less  in  1904  than  all 
the  progeny  from  large  and  small  parents  in  1906, 
a  result  due  to  a  "poor  year"  when  certain  factors 
of  environment  were  unfavorable.  Such  unfavor- 
able conditions,  however,  are  known  to  influence  in 
no  way  the  hereditary  qualities  of  the  beans.  Thus  it 
appears  that,  although  the  progeny  of  a  pure  line 
present  plenty  of  variations  of  the  fluctuating  type, 
due  probably  to  environmental  differences  in  nutri- 
tion, moisture,  etc.,  such  variations  are  quite  inef- 


106 


GENETICS 


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THE   PURE   LINE  107 

fectual  so  far  as  inheritance  is  concerned,  and  it 
makes  no  difference  whether  the  largest  or  the  small- 
est beans  within  a  pure  line  are  selected  from  which 
to  breed,  the  result  will  be  the  same,  in  that  there  is  a 
complete  return  to  mediocrity  or  type  with  no  "in- 
heritance" of  the  parental  modification.  As  a 
matter  of  fact  in  1903,  1906  and  1907  the  lighter 
parents  gave  a  heavier  progeny  than  the  heavier 
parents. 

It  will  be  seen  at  once  that  here  is  a  discovery  of 
far-reaching  importance  which  may  require  us  to 
reconstruct  certain  cherished  ideas  about  the  part 
played  in  the  evolution  of  species,  as  well  as  in 
heredity,  by  natural  selection. 

5.    Cases   similar  to  Johanssen's  Pure  Lines 

Although  according  to  Johanssen  pure  lines  are 
*'the  progeny  of  a  single  self -fertilized  individual," 
it  is  plain  that  in  at  least  three  other  possible  cases 
something  quite  similar  to  "pure  lines"  may  be 
obtained. 

First,  when  two  similar  organisms  identical  in 
their  germinal  determiners  with  regard  to  a  particular 
character  inbreed,  their  progeny  will  form  a  pure  line 
so  far  as  this  particular  character  is  concerned  just  as 
truly  as  two  parents  that  are  united  in  a  single  in- 
dividual produce  a  pure  line  progeny  as  the  result  of 
self-fertilization. 

Second,  in  cases  of  parthenogenesis,  the  progeny 
arising  from  a  single  female  individual  without  the 
customary  qualitative  reduction  of  chromosomes  that 


108  GENETICS 

accompanies  sexual  reproduction,  constitute  a  pure 
line  or  an  unmixed  strain. 

Third,  in  cases  of  asexual  reproduction  where  the 
progeny  are  simply  the  result  of  continued  fission  of 
the  original  individual,  a  pure  line  may  be  said  to 
continue   from   generation   to   generation. 

In  the  second  and  third  categories  it  should  be 
pointed  out  that  the  "pure  line"  is  assured  only  so 
long  as  asexual  reproduction  continues.  It  is  quite 
possible  for  an  organism,  heterozygotic  in  composi- 
tion, to  continue  to  breed  true  or  to  produce  an  ap- 
parently puje  line  so  long  as  asexual  methods  are 
employed.  As  soon  as  such  an  organism,  however, 
changes  to  the  sexual  method  of  reproduction,  seg- 
regation of  characters  may  occur  and  different 
combinations  result. 

6.   Tower's  Potato-beetles 

As  an  illustration  of  the  effect  of  selection  within 
pure  lines  of  the  first  category  may  be  mentioned  a 
case  given  by  Tower  in  his  exhaustive  experiments 
on  the  Colorado  potato-beetle  Leptinotarsa  decem- 
lineata.  Among  the  numerous  cultures  of  this 
beetle  which  were  under  control,  a  considerable 
variation  in  color  made  its  appearance.  For  con- 
venience in  classification  these  variations  wxre 
graded  into  arbitrary  classes  or  graduated  variants 
(see  p.  52)  ranging  from  dark  to  light. 

When  a  male  and  a  female  from  the  extreme  class 
at  the  dark  end  of  the  series  were  allowed  to  breed 
together,  their  progeny  were  not  dark,  but  fluctuated 


Fig,  36.  —  Diagram  showing  the  ineffectiveness  of  selection  through  twelve 
generations  within  a  homozygous  strain  in  the  case  of  the  Colorado 
potato-beetle  (Leptinotarsa).  In  each  generation  extreme  dark  speci- 
mens were  selected  as  the  parents  of  the  succeeding  generation  but  the 
progeny  always  swung  back  to  the  type.    After  Tower. 


no  GENETICS 

in  color  around  the  original  average  of  the  entire 
series.  This  process  of  selecting  each  time  an  ex- 
treme pair  of  dark  parents  was  continued  for  twelve 
generations,  as  shown  in  Figure  36,  without  in  any 
way  increasing  the  percentage  of  brunette  potato 
beetles  in  the  progeny. 

Thus  in  a  pure  line  formed  by  the  breeding  of  two 
individuals  alike  with  respect  to  color,  the  selection 
of  an  extreme  variant  was  quite  without  effect  in 
modifying  the  color  of  the  progeny. 

7.   Jennings'  Work  on  Paramecium 

An  instance  of  the  third  category  of  pure  lines 
is  furnished  by  Jennings'  remarkable  w^ork  on  the 
protozoan  Paramecium,  which  was  published  in  1909. 
Jennings  carried  on  his  experiments  quite  independ- 


206       zoo       194        176        142        US        100 

Fig.  37.  —  Eight  pure  races  of  Paramecium.  The  actual  mean  length  of 
each  race  is  given  in  micra  below  the  corresponding  outline.  Magni- 
fied about  230  diameters.    After  Jennings. 

ently  of  Johanssen,  but  he  nevertheless  arrived  at 
the  same  general  conclusion,  namely,  that  selection 
within  a  pure  line  is  without  effect. 

Jennings  found  that  Paramecia  differ  from  each 


THE   PURE   LINE  111 

other  in  size,  structure,  physical  character,  and  rate 
of  niultiphcation  as  well  as  in  the  environmental 
conditions  required  for  their  existence  and,  further- 
more, that  these  differences,  in  an  hereditary  sense, 
are  "as  rigid  as  iron." 

With  respect  to  the  character  of  mean  length  he 
was  able  to  isolate  eight  races,  or  pure  lines,  whose 
average  size,  drawn  to  scale,  is  shown  in  Figure  37. 


256   < MICRA  > 

Fig.  38.  — Diagram  of  a  single  race  (D)  showing  the  variation  in  the  size 
of  the  individuals.     Magnified  about  230  diameters.    After  Jennings. 

Each  of  these  pure  lines  produced  a  progeny 
which  exhibited  a  considerable  range  of  fluctuating 
variation.  The  offspring  of  pure  line  D,  for  example, 
varied  from  ^56  to  80  micra  ^  in  length  with  an  aver- 
age of  176  micra,  as  shown  in  Figure  38,  where  samples 
of  the  different  classes  of  variants  in  pure  line  D  are 
arranged  in  a  series. 

A  single  representative  of  each  of  the  different 
classes  of  variants  out  of  all  of  the  eight  pure  lines 
bred  by  Jennings  is  shown  in  Figure  39. 

Each  horizontal  row  represents  a  single  race  or 

^  A  micron  is  jo^o^th  of  a  millimeter. 


112 


GENETICS 


pure  line,  the  average  size  of  which  is  indicated  by  the 
sign  + .     The  mean  length  of  the  entire  lot,  as  shown 

J  55 


DOOHa 


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aO'OHHa 


8()898eH08M 


gyflHH  98 « » » 


gyjHMM  89  » e 


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M  M  «  «  «  « 

45 


Fig.  39.  —  Diagram  of  the  species  Paramecium  as  made  up  of  the  eight 
different  races  shown  in  Figure  37.  Each  horizontal  row  represents  a 
single  race.  The  individual  showing  the  mean  size  in  each  race  is  in- 
dicated by  a  cross  placed  above  it.  The  mean  for  the  entire  lot  is  at 
the  horizontal  line.  The  magnification  is  about  24  diameters.  After 
Jennings. 

by  the  vertical  line,  is  155  micra.  The  total  number 
of  individuals  belonging  to  each  size  is  not  indicated, 
but  in  every  horizontal  line  their  number  is  more 


THE   PURE   LINE  113 

numerous  near  the  average  for  that  line  and  less 
numerous  at  the  extremes,  thus  forming  the  typical 
normal  frequency  polygons  of  fluctuating  variability. 
The  significant  fact  about  these  series  is  this,  that 
extreme  individuals  selected  from  any  pure  line  do 
not  reproduce  extreme  sizes  like  themselves,  but 
instead,  a  progeny  varying  according  to  the  laws  of 
chance  around  the  average  standard  of  the  particular 
line  from  which  it  came. 

8.   Phenotypical  and  Genotypical  Distinctions 

From  the  foregoing  it  will  be  seen  that  the  be- 
havior of  an  organism  in  heredity  cannot  always 
be  determined  by  an  inspection  of  its  somatic  char- 
acters alone. 

For  example,  six  Paramecia,  each  155  micra  in 
length  and  apparently  identical,  could  be  selected 
from  the  six  upper  pure  lines  in  Jennings'  table  given 
in  Figure  39  which  would  produce  six  progenies 
definitely  unlike,  whereas  in  the  case  of  pure  line  D, 
twenty-four  Paramecia,  all  measurably  different 
from  each  other  in  size,  would  be  found  to  produce 
twenty-four  progenies  practically  identical. 

Organisms  that  appear  to  be  alike,  regardless  of 
their  germinal  constitution,  are  said  by  Johanssen 
to  be  identical  phenotypically ,  or  to  belong  to  the 
same  phenotype. 

On  the  other  hand,  organisms  having  identical 
germinal  determiners  such  as  those  of  the  varying 
members  of  pure  line  /),  are  said  to  be  genotypically 
alike  or  to  belong  to  the  same  genotype. 


114  GENETICS 

Organisms  belong  to  the  same  phenotype  with 
respect  to  any  character  when  their  somatoplasms 
are  alike.  They  belong  to  the  same  genotype  when 
their  germplasms  are  alike. 

The  word  "genotype"  was  suggested  by  Johanssen 
in  honor  of  Darwin  and  his  theory  of  psmgenesis, 
although  there  are  certain  objections  to  its  use  in 
this  connection  for  the  reason  that  systematists  have 
already  appropriated  it  in  a  different  sense. 

Natural  history  and  common  usage  deal  prin- 
cipally with  phenotypes,  that  is,  with  organisms  as 
they  appear.  The  older  theories  of  heredity  were 
likewise  concerned  wdth  phenotypes,  but  we  are  now 
coming  to  see  more  clearly  than  before  that  heredity 
must  always  be  a  case  of  similarity  in  origin,  that  is, 
in  germinal  composition,  and  that  similarity  in  ap- 
pearance by  no  means  always  indicates  similarity 
in  origin  or  true  relationship. 

The  assumption  that  similarity  in  appearance  does 
indicate  relationship  has  been  made  the  foundation 
of  many  conclusions  in  comparative  anatomy  and 
phylogeny,  but  to  the  modern  student  of  genetics 
who  places  his  faith  in  things  as  they  are,  rather  than 
in  things  as  they  seem  to  be,  conclusions  based  upon 
phenotypical  distinctions  alone  have  in  them  a  large 
source  of  error  which  must  be  taken  into  account. 

In  a  museum  of  heredity,  should  such  a  collection 
ever  be  assembled,  the  specimens  would  not  be  ar- 
ranged phenotypically  as  they  are  in  an  ordinary 
museum  where  things  that  look  alike  are  placed 
together   as   if   in   bonds   of   relationship,    but   they 


THE   PURE   LINE  115 

would  be  arranged  historically  from  a  genetic  point 
of  view  to  show  their  true  origin  one  from  another. 

9.   The  Distinction  between  a  Population 

AND  A  Pure  Line 

A  mixture  of  pure  lines  has  been  called  a  popula- 
tion. 

It  is  not  possible  to  distinguish  a  pure  line  from  a 
population  by  inspection,  since  both  may  be  pheno- 
typically  alike.  Fluctuations  about  the  average 
occur  in  both  cases  with  no  appreciable  difference 
in  character,  although  such  fluctuations,  when  they 
occur  within  a  pure  line,  are  simply  somatic  differ- 
ences caused  in  general  probably  by  modifications 
in  nutrition  or  some  other  external  factor  of  environ- 
ment, while  fluctuations  in  a  population  include  not 
only  modifications  of  this  transient  nature,  but  also 
permanent  hereditary  differences  due  to  germinal 
differences  in  the  various  pure  lines  of  which  the 
population  is  composed. 

Johanssen  has  made  the  distinction  between  pure 
lines  and  populations  clear  by  the  following  figure 
(Fig.  40),  in  which  five  pure  lines  of  beans  are  com- 
bined artificially  to  form  a  population. 

The  beans  which  make  up  the  pure  lines  noted  in 
this  figure  are  represented  inclosed  within  inverted 
test  tubes.  The  beans  in  any  single  tube  are  all  of 
one  size.  Tubes  vertically  superimposed  upon  each 
other  also  contain  only  beans  of  one  size. 

Thus  it  is  seen  that  what  may  be  a  rare  size  of 
bean  in  one  line,  for  instance  that  in  the  left-hand 


116 


GENETICS 


n 


^  igji^^iji  -^  ^  ^  -j^jji 


tube  of  jpure  line  3,  may  be  identical  with  the  com- 
monest size  in  another  line,  as  jpure  line  2.      The 
Pure:  Line  five    pure    lines 

represented  in 
Figure  40  are 
combined  in  a 
population  at  the 
bottom  of  the 
figure^  making 
a  phenotype 
that  marks  the 
five  phenotypes 
above,  which  are 
also  five  geno- 
types. In  the 
population,  how- 
ever, the  five 
genotypes  are 
hidden  within 
one  phenotype. 
Hence,  while 
selection  within 
a  pure  line  has 
no  hereditary  in- 
fluence, it  is  evi- 
dent that  selec- 
tion within  a 
population  may 
shift  or  move 
over  the  type 
of   the   progeny 


Population 


HI 

Fig.  40.  —  Diagrams  showing  five  jyure  lines 
and  a  population  formed  by  their  union.  The 
beans  of  each  pure  line  are  represented  as  as- 
sorted into  inverted  test  tubes  making  a  curve 
of  fluctuating  variability.  Test  tubes  contain- 
ing beans  of  the  same  weight  are  placed  in  the 
same  vertical  row.    After  Johanssen. 


THE   PURE   LINE  117 

obtained,  in  the  direction  of  the  selection  simply 
by  isolating  out  a  pure  line  of  one  type.  Thus 
beans  chosen  from  the  extreme  left-hand  test  tube 
in  the  population  cited  would  belong  only  to  'pure 
line  2,  while  those  taken  from  the  extreme  right- 
hand  test  tube  could  belong  only  to  pure  line  3. 

Galton's  "law  of  regression,"  namely,  that  minus 
parents  give  minus  offspring  and  plus  parents  plus 
offspring,  with  a  tendency  to  reversion  from  genera- 
tion to  generation,  depends  simply  upon  a  partial 
but  not  complete  isolation  of  pure  lines  out  of  a 
population. 

From  this  distinction  between  pure  lines  and  popu- 
lations it  is  clear  why  breeders  in  selecting  for  a 
particular  character  out  of  their  stock  need  to  keep 
on  selecting  continually  in  order  to  maintain  a  cer- 
tain standard.  As  soon  as  they  cease  this  vigilance, 
there  is  a  "reversion  to  type"  or,  as  they  say,  "the 
strain  runs  out,"  which  means  that  the  pure  lines 
become  lost  in  the  mixed  population  which  inevi- 
tably results  as  soon  as  selective  isolation  of  the  pure 
line  ceases. 

Such  reversion  must  always  be  the  case  in  dealing 
with  a  population  made  up  of  a  mixture  of  pure 
lines,  for  only  by  the  isolation  of  pure  lines  can 
the  constancy  of  a  character  be  maintained.  When, 
however,  a  pure  line  is  once  isolated,  then  all  the  mem- 
bers of  it,  large  as  well  as  small,  are  equally  efficient 
in  maintaining  the  pure  line  in  question,  regardless 
of  their  phenotypical  constitutions. 


118  GENETICS 

10.   Pure  Lines  and  Natural  Selection 

From  the  foregoing  statements  it  appears  that  by 
means  of  selection  within  a  population,  such  as  occurs 
normally  in  nature,  it  is  not  possible  to  get  anything 
out  that  was  not  already  there  to  begin  with.  If 
this  is  so,  the  origin  of  species  cannot  have  come 
about,  as  Darwin  thought,  through  natural  selection 
by  a  gradual  accumulation  of  slight  favorable  varia- 
tions. The  best  that  selection  can  do  is  to  isolate 
pure  lines.  Within  pure  lines  it  is  quite  powerless 
to  change  the  genet ypical  characters.  In  other 
words,  natural  selection  can  only  maintain  and 
strengthen  the  frontier  posts  that  are  already  es- 
tablished. It  cannot  break  into  the  wilderness  and 
create  new  centers. 

Since  the  extreme  members  of  a  pure  line,  having 
the  same  genotypical  constitution,  always  tend  to 
backslide  to  mediocrity  within  the  limits  of  the  line 
in  question,  the  crucial  question  is :  How  can  the 
critical  step  from  one  genotype  to  another,  a  step 
indispensable  in  the  evolutionary  derivation  of 
species,  ever  occur  ?  That  it  has  repeatedly  oc- 
curred in  the  course  of  time  is  amply  proven  by  the 
fact  that  somehow  or  other  we  have  gone  from 
Ameba  to  man. 

At  present  the  only  loophole  of  escape  seems  to  lie 
either  in  the  unlikely  inheritance  of  acquired  char- 
acters, or  in  mutations  which  make  the  leap  from 
one  character  to  another,  and  so  eventually"  from  one 
type  to  another,  without  the  aid  of  selection. 


THE   PURE   LINE  119 

It  is  interesting  to  note  that  Johanssen  himself, 
who  has  been  so  prominently  concerned  in  erecting 
this  barrier  in  the  way  of  the  evolutionary  derivation 
of  species  by  natural  selection,  has  recently  reported 
mutations  arising  within  his  pure  lines  of  beans. 
It  must  be  admitted  that  to  the  skeptical  there  is  a 
vicious  circle  here,  for  when  a  variation  fails  to  re- 
appear in  a  subsequent  generation,  it  may  be  ex- 
plained as  the  failure  of  natural  selection  to  act 
within  a  pure  line,  but  when  a  variation  does  reap- 
pear it  is  hailed  as  a  mutation! 

In  any  event  the  way  of  experiment  lies  open, 
and  the  evidence  of  investigators  in  this  critical 
field  will  be  awaited  with  keen  interest. 


CHAPTER  VII 

SEGREGATION  AND  DOMINANCE 
1.   Methods  of  Studying  Heredity 

Modern  studies  in  heredity  have  been  pursued 
principally  in  three  directions  :  first,  by  microscopical 
examination  of  the  germ-cells ;  second,  by  statistical 
consideration  of  data  hearing  upon  heredity ;  and 
third,  by  experimental  breeding  of  animals  and 
plants. 

The  first  two  of  these  methods  of  approach  have 
already  been  touched  upon  as  well  as  experimental 
breeding  with  reference  to  "pure  lines."  In  the 
present  chapter  attention  will  be  directed  to  a  con- 
sideration of  experimental  breeding  with  reference 
to  hybridization,  that  is,  breeding  from  unlike  par- 
ents, a  process  which  Jennings  characterizes  by  the 
expressive  phrase,  "the  melting-pot  of  cross-breed- 
mg. 

2.   The  Melting-pot  of  Cross-breeding 

Hybridization,  or  cross-breeding,  as  formulated 
by  Galton  (1888),  results  in  one  of  three  kinds  of 
inheritance,  namely,  blending,  alternative,  or  par- 
ticulate. 

120 


SEGREGATION  AND   DOMINANCE         121 

Of  these,  blending  inheritance  may  be  called  the 
typical  "melting-pot"  in  which  contributions  from 
the  two  parents  fuse  into  something  intermediate 
and  different  from  that  which  was  present  in  either 
parent.  Galton  illustrated  this  process  by  the 
inheritance  of  human  stature  in  which  a  tall  and 
a  short  parent  produce  offspring  intermediate  in 
height.  A  more  thorough  consideration  of  this  type 
of  inheritance  will  be  presented  in  Chapter  IX. 

By  the  method  of  alternative  inheritance  the  pa- 
rental contributions  do  not  melt  upon  union,  but 
retain  their  individuality,  reappearing  intact  in  the 
offspring.  In  inheritance  of  human  eye-color,  for  ex- 
ample, the  offspring  usually  have  eyes  colored  like 
those  of  one  of  the  parents  when  the  parental  eye- 
color  is  unlike  in  the  two  cases,  rather  than  eyes 
intermediate  in  color  between  those  of  both  parents. 

According  to  Galton  particulate  inheritance  results 
when  the  offspring  present  a  mosaic  of  the  parental 
characters,  that  is,  when  parts  of  both  the  maternal 
and  paternal  characters  reappear  in  the  offspring 
without  losing  their  identities  by  blending  or  without 
excluding  one  another.  Piebald  races  of  mice  arising 
from  parents  with  solid  but  different  colors  have 
been  cited  as  illustrations  of  this  sort  of  inheritance, 
although  it  will  be  seen  later  in  connection  with  the 
''factor  hypothesis"  that  another  interpretation  of 
this  phenomenon  is  not  only  possible  but  probable. 

The  distinctions  between  these  three  categories 
of  inheritance  are  diagrammatically  represented  in 
Figure  41. 


122 


GENETICS 


In  blending  inheritance  the  offspring  are  seen  to 
be  unhke  either  parent,  because  the  parental  deter- 
miners fuse  into  a  new  thing.  In  alternative  in- 
heritance, on  the  contrary,  the  offspring  may  be 
like  either  parent,  since  the  characters  in  question  do 
not  lose  their  individuality  upon  union,  as  shown  in 
the   diagram.     Only   one   or  the   other   of  the   two 


Blending 


Alternative 


Particulate 


Characteristics 
of    parental 
jermplasm  as 
Shovun  in  the 
Somaplasm 


Double  germplasm 
termed    from 
Contributions 
from  both  parents 


Possible    kind* 
of  apparent 

dfsprinj    this 

jermplasm 

maif  produce 


Fig.  41.  —  Three  kinds  of  inheritance  described  by  Galton. 


mutually  exclusive  characters  thus  becomes  effective 
in  determining  the  nature  of  each  offspring. 

Finally,  in  particulate  inheritance  the  double 
germplasm  which  determines  a  new  individual  may 
be  imagined  to  undergo  a  diagonal  rather  than  a 
vertical  cleavage  upon  maturation,  thereby  causing 
unblended  fragments  of  both  parental  characters 
to  become  effective  at  once,  in  this  manner  producing 
a  mosaic  offspring. 


SEGREGATION   AND   DOMINANCE         123 

3.     JOHANN    GrEGOR    MeNDEL 

Our  understanding  of  the  working  of  inheritance 
in  hybridization  we  owe  largely  to  the  unpretentious 
studies  of  an  Austrian  monk,  Johann  Gregor  Mendel, 
who,  although  a  contemporary  of  Darwin,  was  prob- 
ably unknown  to  him.  For  eight  years  Mendel 
carried  on  original  experiments  by  breeding  peas  in 
the  privacy  of  his  cloister  garden  at  BrUnn  and  then 
sent  the  results  of  his  work  to  a  former  teacher, 
the  celebrated  Karl  Nageli,  of  the  University  of 
Vienna.  At  the  time  Nageli's  head  was  full  of  other 
matters,  so  that  he  failed  to  see  the  significance  of 
his  old  pupil's  efforts.  However,  in  188[6  Mendel's 
results  appeared  in  the  Transactions  of  the  Natural 
History  Society  of  Brunn,^  an  obscure  publication 
that  reached  hardly  more  than  a  local  public.  Here 
Mendel's  investigations  were  buried,  so  to  speak, 
because  the  time  was  not  ripe  for  a  general  apprecia- 
tion or  evaluation  of  his  work. 

At  that  time  neither  the  chromosome  theory  nor 
the  germplasm  theory  had  been  formulated.  More- 
over, much  of  our  present  knowledge  of  cell  structure 
and  behavior  was  not  even  in  existence.  Weismann 
had  not  yet  led  out  the  biological  children  of  Israel 
through  the  wilderness  upon  that  notable  pilgrimage 
of  fruitful  controversy  which  occupied  the  last  two 
decades  of  the  nineteenth  century,  and  the  attention 
of  the  entire  thinking  world  was  being  monopolized 

1  Verhandlungen  naturf.  Verein  in  Briinn.  Abhandl.  IV,  1865  (which 
appeared  in  18G6). 


124  GENETICS 

by  the  newly  published  epoch-making  work  of  Charles 
Darwin. 

Mendel  died  in  1884,  and  his  work  slumbered  on 
until  it  was  independently  discovered  almost  simul- 
taneously by  three  botanists  whose  researches  had 
been  leading  up  to  conclusions  very  much  like  his 
own.  These  three  men  were  de  Vries  of  Holland, 
von  Tschermak  of  Austria,  and  Correns  of  Germany. 
Their  papers  were  published  only  a  few  months  apart 
in  1900  and  were  closely  followed  by  important 
papers  from  Bateson  in  England  and  Davenport 
and  Castle  in  America,  with  a  rapidly  increasing 
number  from  other  biologists  the  world  over.  To- 
day the  literature  upon  this  subject  has  grown  to 
be  very  large,  and  the  end  is  by  no  means  yet  in 
sight. 

Concerning  Mendel,  Castle  has  well  said:  "Mendel 
had  an  analytical  mind  of  the  first  order  which  en- 
abled him  to  plan  and  carry  through  successfully 
the  most  original  and  instructive  series  of  studies 
in  heredity  ever  executed." 

4.   Mendel's  Experiments  on  Garden  Peas 

What  Mendel  did  was  to  hybridize  certain  varie- 
ties of  garden  peas  and  keep  an  exact  record  of  all 
the  progeny,  in  itself  a  simple  process  but  one  that 
had  never  been  faithfully  carried  out  by  any  one. 

Before  examining  Mendel's  results  it  may  be  well 
to  state  the  difference  between  normal  and  artificial 
self-fertilization.  Self-fertilization  occurs  when  from 
the  pollen  and  ovule  of  the  same  flower  are  derived 


SEGREGATION  AND   DOMINANCE         125 

the  two  gametes  which  uniting  produce  a  zygote 
that  develops  into  the  seed  and  subsequently  into 
the  adult  plant  of  the  next  generation.  In  artifi- 
cially crossing  normally  self-fertilized  flowers  it  is 
necessary  to  carefully  remove  the  stamens  from  one 
flower  while  its  pollen  is  still  immature,  and  later,  at 
the  proper  time,  to  transfer  to  it  ripe  pollen  from 
another  flower. 

Mendel's  cross-breeding  experiments  on  peas 
showed  certain  numerical  relations  which  gave  rise 
to  what  has  come  to  be  rather  indefinitely  known  as 
"Mendel's  law."  This  law  may  be  temporarily 
formulated  as  follows  :  — 

When  parents  that  are  unlike  with  respect  to  any 
character  are  crossed,  the  progeny  of  the  first  gen- 
eration will  apparently  be  like  one  of  the  parents 
w4th  respect  to  the  character  in  question.  The 
parent  which  impresses  its  character  upon  the  off- 
spring in  this  manner  is  called  the  dominant.  When, 
however,  the  hybrid  offspring  of  this  first  generation 
are  in  turn  crossed  with  each  other,  they  will  produce 
a  mixed  progeny,  25  per  cent  of  which  will  be  like  the 
dominant  grandparent,  25  per  cent  like  the  other 
grandparent,  and  50  per  cent  like  the  parents  resem- 
bling the  dominant  grandparent. 

An  illustration  will  serve  to  make  plain  the  man- 
ner in  which  this  law  works  out. 

Mendel  found  that  when  peas  of  a  tall  variety 
were  artificially  crossed  with  those  of  a  dwarf  variety, 
all  the  resulting  offspring  were  tall  like  the  first 
parent.     It  made  no   difference   which  parent   was 


126  GENETICS 

selected  as  the  tall  one.  The  result  was  the  same 
in  either  case,  showing  that  the  character  of  tallness 
is  independent  of  the  character  for  sex. 

When  these  tall  cross-bred  offspring  were  subse- 
quently crossed  with  each  other,  or  allowed  to  pro- 
duce offspring  by  self-fertilization  which  amounts 
to  the  same  thing,  787  plants  of  the  tall  variety  and 
277  of  the  dwarf  kind  were  obtained,  making  approx- 
imately the  proportion  of  3  to  1. 

On  further  breeding  the  dwarf  peas  thus  derived 
proved  to  be  pure,  producing  only  dwarf  peas,  while 
the  tall  ones  were  of  two  kinds,  one  third  of  them 
"pure,"  breeding  true  like  their  tall  grandparent, 
and  two  thirds  of  them  "hybrid,"  giving  in  turn  the 
proportion  of  three  tall  to  one  dwarf  like  their  parents. 

These  crosses  may  be  expressed  as  follows :  — 

Tall,  r,  X  dwarf,  U  =  tall,  T(t). 

That  is,  tallness  crossed  with  dwarfness  equals 
tallness  with  the  dwarf  character  present  but  latent. 

Mendel  termed  the  character,  which  became  ap- 
parent in  such  a  hybrid,  in  this  case  tallness,  the 
dominant,  and  the  latent  character  which  receded 
from  view,  in  this  instance  dwarfness,  the  recessive. 

When  now  the  hybrids,  T(t),  were  crossed  to- 
gether, the  result  algebraically  expressed  was  as 
follows  :  — 

T  -{- 1  (all  possible  egg  characters) 
T-\-t  (all  possible  sperm  characters) 
TT+     Tt 

Tt     +tt 

TT+2T(t)+tt 


SEGREGATION  AND   DOMINANCE 


127 


That  is,  one  out  of  four  possible  cases  was  dwarf,  t, 
in  character  and  the  other  three  were  apparently 
tall,  although  only  one  out  of  the  three  was  pure  tall, 
T,  while  the  remaining  two  were  tall  with  the  dwarf 
character  latent,  T  (t). 

The  same  thing  may  be  expressed  more  graphically 
by  the  checkerboard  plan. 


Male   Gametes 

T       I 


to 


^.T 


which  Punnett  suggested 
(Fig.  42).  Each  square 
of  the  checkerboard  rep- 
resents a  zygote  which, 
having  received  a  gamete 
from  each  of  the  two  par- 
ents, may  develop  into  a 
possible  offspring.  The 
character  of  the  gametes 
of  the  parents  is  shown 
outside  of  these  squares, 
while  the  arrows  repre- 
sent the  parental  source 
from  which  the  offspring  have  received  their  heredi- 
tary composition. 

The  essential  feature  of  Mendel's  law  is  briefly 
this:  hereditary  characters  are  usually  independent 
units  which  segregate  out  upon  crossing,  regardless  of 
temporary  dominance. 

Mendel  carried  on  further  experiments  with  garden 
peas,  using  other  characters.  He  obtained  practically 
the  same  result  as  in  the  instance  already  given,  for 
the  actual  progeny  in  the  second  generation  of  the 
cross-bred  offspring  figured  up,  as  seen  in  the  table 


z 
< 

la 

< 

Z 
u 


Fig.  42. — Diagram  to  illustrate 
theoretically  the  formation  of  the 
four  possible  zygotes  in  the  second 
filial  generation  of  a  monohybrid. 


128 


GENETICS 


below,  very  nearly  to  the  expected  theoretical  ratio 
of  3  to  1. 


Character 

Number  of 
Dominants 

Number  of 
Recessives 

Ratio 

Form  of  seed      .... 

5474  smooth 

1850  wrinkled 

2.96  to  1 

Color  of  seed  coat 

6022  yellow 

2001  green 

3.01  to  1 

Color  of  flowers 

705  colored 

224  white 

3.15  to  1 

Form  of  pods     .     . 

882  inflated 

299  constricted 

2.95  to  1 

Color  of  unripe  pods 

428  green 

152  yellow 

2.82  to  1 

Position  of  flowers 

651  axial 

207  terminal 

3.14  to  1 

Length  of  vine 

787  tall 

277  dwarf 

2.84  to  1 

Total     .... 

2.98  to  1 

Darbyshire  repeated  the  yellow-green  cross  with 
garden  peas,  obtaining  in  the  second  generation  the 
large  total  of  139,837  individuals  of  which  105,045 
were  yellow  and  34,792  green,  wliich  is  very  close  to 
3  to  1. 


o. 


Some  Further  Instances  of  "Mendel's 

Law  " 


Since  the  rediscovery  of  Mendel's  law  the  ratio  of 
3  to  1  in  the  second  generation  has  been  found  by 
a  number  of  different  investigators  to  be  constant  in 
a  large  array  of  characters  observed  both  in  animals 
and  plants  of  diverse  kinds  wdien  these  are  cross-bred 
with  reference  to  the  characters  in  question. 

Botanists  have  the  advantage  perhaps  in  this 
matter,  as  they  deal  with  forms  which  usually  produce 
a  large  number  of  offspring  from  a  single  cross,  — 
a  very  desirable  thing  in   estimating  ratios.     On  the 


SEGREGATION  AND   DOMINANCE 


1^29 


other  hand,  they  are  handicapped  by  being  unable 
usually  to  obtain  more  than  one  generation  in  a  year, 
while  zoologists  may  secure  from  many  animals  like 
rabbits  and  mice  several  generations  in  a  year,  al- 
though ordinarily  the  number  of  progeny  is  much 


Organism 

Author 

Dominant 

Recessive 

P 

Nettles 

Correns 

Serrated  leaves 

Smooth-margined  leaves 

'03 

Sunflower 

ShuU 

Branched  habit 

Unbranched  habit 

•08 

Cotton 

Balls 

Colored  lint 

White  lint 

'07 

Snapdragon 

Baur 

Red  flowers 

Non-red  flowers 

'10 

Wheat 

Biff-en 

Susceptibility     to 
rust 

Immunity  to  rust 

'05 

Tomato 

Price    and 
Drinkard 

Two-celled  fruit 

Many-celled  fruit 

'08 

Maize 

de  Vries 

Round,        starchy 
kernel 

Wrinkled,  sugary  kernel 

'00 

Silkworm 

Toyama 

Yellow  cocoon 

White  cocoon 

'06 

Cattle 

Spillman 

Hornlessness 

Horns 

'06 

Pomace  fly 

Alorgan 

Red  eyes 

White  eyes 

•10 

Horses 

Bateson 

Trotting  habit 

Pacing  habit 

'07 

Land  snail 

Lang 

Unbanded  shell 

Banded  shell 

'09 

Mice 

Darbyshire 

Normal  habit 

Waltzing  habit 

'02 

Guinea-pig 

Castle 

Short  hair 

Angora  hair 

'03 

Canaries 

Bateson  and 
Saunders 

Crest 

Plain  head 

'02 

Poultry- 

Davenport 

Rumplessness 

Long  tail 

•06 

Man 

Farrabee 

Brachydactyly 

Normal  joints 

'05 

Barley 

von  Tsehermak 

Beardlessness 

Beardedness 

'01 

Salamander 

(Amblystoma) 

Haecker 

Dark  color 

Light  color 

•08 

smaller  and  the  ratios  obtained  have  a  larger  chance 
of  error  than  is  the  case  with  the  more  numerous 
plant  offspring. 

Semi-microscopic  animals,  as,  for  example,  the 
pomace  fly,  Drosojphila,  which  produces  a  large  progeny 
every  two  weeks  or  so,  may  combine  the  general 
advantages  mentioned  for  the  two  groups  of  organisms 


130  GENETICS 

indicated  above,  but  they  have  the  disadvantage  of 
being  so  small  that  the  detection  of  their  distinctive 
phenotypic  characters  is  attended  with  considerable 
technical  difficulty. 

What  the  modern  experimenter  in  genetics  desires  is 
an  organism,  first,  that  possesses  conspicuous  distinc- 
tive somatic  characters,  and,  second,  which  w411  come  to 
sexual  maturity  early  and  breed  either  in  captivity 
or  under  cultivation  both  numerously  and  frequently. 

The  preceding  table,  compiled  chiefly  from  Bateson  ^ 
and  Baur,-  might  easily  be  much  extended.  It  shows 
from  what  diverse  sources  confirmatory  evidence  of  the 
truth  of  Mendel's  law  has  been  derived  within  the 
past  few  years. 

6.   The  Principle  of  Segregation 

The  essential  thing  which  Mendel  demonstrated 
was  the  fact  that,  in  certain  cases  at  least,  the  deter- 
miners for  heredity  derived  from  diverse  parental 
sources  may  unite  in  a  common  stream  of  germplasm 
from  which,  in  subsequent  generations,  they  may 
segregate  out  apparently  unmodified  by  having  been 
intimately  associated  with  each  other.  This  "law  of 
segregation"  depends  upon  the  conception  that  the 
individual  is  made  up  of  a  bundle  of  unit  characters. 
It  may  be  illustrated  by  the  separate  flowers  picked  ^ 
from  a  garden  which,  after  being  made  into  a  nose- 
gay, may  be  taken  apart  and  rearranged  without  in 

1  "Mendel's  Principles  of  Heredity,"  1909. 

'^  "  Einf  iihrung  in  die  experimentelle  Vererbungslehre,"  1911. 


SEGREGATION  AND   DOMINANCE         131 

any    way    disturbing    the    identity    of   the    separate 
blossoms. 

The  general  formula  of  segregation  that  covers 
all  cases  of  organisms  cross-bred  \\ath  respect  to  a 
single  character,  that  is,  monohybrids,  is  given  in 
Figure  43. 

U  (Dominant)  K  (Recesiln) 


D(R) 
1 


DD      2  D(R)      RR 

i 

OD        DD      2D(R)       RR         RR 

^ 1 

DD       uf     ' 

Fig.  43.  —  General  Mendelian  formula  for  a  monohybrid. 
7.     HOMOZYGOTES   AND   HeTEROZYGOTES 

A  character  which  is  present  in  the  offspring  in 
double  quantity  because  it  was  present  in  both  parents 
is  said  by  Bateson  to  be  homozygous,  while  an  or- 
ganism which  is  homozygous  with  respect  to  any 
character  is  called  a  hoinozygote  so  far  as  that  particu- 
lar character  is  concerned. 

In  contrast  to  the  homozygous  condition,  an  organ- 
ism is  said  to  be  heterozygous  when  it  derives  the 
determiner  of  a  character  from  one  parent  only. 
Such  an  organism  is  described  as  a  heterozygote  with 
respect  to  the  character  in  question.  A  homozygous 
and    a    heterozygous    dominant    may  appear    alike. 


132  GENETICS 

although  not  necessarily  so,  that  is,  they  may  have 
the  same  phenotypical  constitution,  but  their  geno- 
typical  composition  is  always  different. 

8.  The  Identification  of  a  Heterozygote 

"Homozygote"  and  "heterozygote"  are  terms  then 
descriptive  solely  of  the  genotypical  constitution  of 
organisms,  and,  as  has  been  said,  it  is  not  always 
possible  to  distinguish  one  from  the  other  by  inspec- 
tion, although  it  may  frequently  be  done,  as  will  be 
pointed  out  later.  The  only  sure  way  to  identify  a 
heterozygote  is  by  breeding  to  a  recessive  and  observing 
the  kind  of  offspring  produced. 

Peas  of  the  formulae  TT  and  T{t),  for  example, 
both  look  alike,  since  a  single  determiner  for  the  tall 
character,  T,  is  sufficient  to  produce  complete  tallness. 
When,  however,  these  two  kinds  of  tall  peas  are 
each  bred  to  a  recessive  dwarf  pea,  of  the  formula  tt, 
the  progeny  will  differ  distinctly  in  the  two  cases  as 
follows :  — 

Case  I.     r  +  r  X  <  +  /  =  100  per  cent  ^(0. 
Case  II.     T  +  tXt  +  t  =  oOper  cent  1(1)  +  50  per  cent  ti. 

That  is,  if  the  dominant  to  be  tested  is  homozygous 
(Case  I),  the  entire  progeny  will  exhibit  the  dominant 
character,  but  if  the  dominant  to  be  tested  is  heterozy- 
gous (Case  II),  then  only  one  half  of  the  progeny  will 
show  the  character  in  question. 

9.  The  Presence  and  Absence  Hypothesis 

Mendel's  conception  that  every  dominant  character 
is  paired  with  a  recessive  alternative  is  now  being 


SEGREGATION  AND   DOMINANCE         133 

largely  replaced  by  the  ^presence  and  absence  hypothesis 
which  was  first  suggested  by  Correns  but  later  logi- 
cally worked  out  by  others,  particularly  by  Hurst, 
Bateson,  and  Shull.  According  to  this  latter  inter- 
pretation, a  determiner  for  any  character  either  is, 
or  is  not,  present.  When  it  is  present  in  two  parents, 
then  the  offspring  receive  a  double,  or  duplex,  *'dose," 
to  use  Bateson's  word,  of  the  determiner.  When  it 
is  present  in  one  parent  only,  then  the  offspring  have 
a  single,  or  simplex,  dose  of  the  character.  When  it 
is  present  in  neither  parent,  it  follows  that  it  will  not 
appear  in  the  offspring.  In  this  case  the  offspring 
are  said  to  be  nulliplex  wdth  respect  to  the  char- 
acter in  question.  Take  the  case  of  tall  and  dwarf 
peas,  the  determiner  for  tallness  when  present  pro- 
duces tall  peas,  even  if  it  comes  from  one  parent 
only,  but  if  this  determiner  for  tallness  is  absent  from 
both  parents,  the  offspring  are  nulliplex,  that  is,  the 
absence  of  tallness  results  and  only  dwarf  peas  are 
produced. 

The  difference  between  the  presence  and  absence 
theory  and  the  dominant  and  recessive  theory  is  that 
in  the  former  case  the  ''recessive"  character  has  no 
existence  at  all,  while  in  the  latter  instance  it  is 
present,  but  in  a  latent  condition. 

10.     DiHYBRIDS 

So  far  reference  has  been  made  exclusively  to  mono- 
hybrids,  any  two  of  which  are  supposed  to  be  similar 
except  with  respect  to  a  single  unit  character.  ]\Iono- 
hybrids    are    comparatively    simple,    but    when    two 


134  GENETICS 

organisms  are  crossed  which  differ  from  each  other 
with  respect  to  two  different  unit  characters,  the  situa- 
tion becomes  more  comphcated. 

Mendel  solved  the  problem  of  dihybrids  by  crossing 
wrinkled-green  peas  with  smooth-yellow  peas.  He 
found  that  smoothness  S  is  dominant  over  wrinhled- 
ness  W  and  that  yellow  color  Y  is  dominant  over 
green  G,  or,  as  it  would  be  stated  according  to  the 
presence  and  absence  theory,  smoothness  is  a  positive 
character  which  fills  out  the  seed-coat  to  plumpness 
while  its  absence  leaves  a  wrinkled  coat,  and  yellow- 
ness is  a  positive  character  due  to  a  fading  of  the  green 
which  causes  the  yellow  to  be  apparent.  In  the 
absence  of  this  green  fading  factor  or  determiner  the 
green,  of  course,  appears. 

If  smooth-yellow  SY  and  wrinkled-green  WG  are 
crossed,  all  the  offspring  are  smooth-yellow,  but 
they  carry  concealed  the  recessive  determiners  for 
wrinkledness  and  greenness  according  to  the  formula 
S{W)Y(G).  When  the  determiners  of  these  cross- 
breds  segregate  out  during  the  maturation  of  the 
germ-cells,  they  may  recombine  so  as  to  form  four 
possible  double  gametes,  namely,  smooth-yellow  SY 
and  wrinkled-green  WG,  which  are  exactly  like  the 
grandparental  determiners  from  which  they  arose, 
and  in  addition,  two  entirely  new  combinations, 
smooth-green  SG  and  wrinkled-yellow  TT^F. 

Since  the  male  and  the  female  cross-breds  are  each 
furnished  with  these  four  possible  gametic  combina- 
tions, the  possible  number  of  zygotes  formed  by  their 
union   will    be    sixteen    (4x4  =  16).     That    is,   the 


SEGREGATION   AND   DOMINANCE        135 

monohybrid  proportion  of  3  to  1  in  tlihybrid  com- 
binations is  squared,  (3  +  1)-  =  16. 

It  of  course  does  not  follow  that  the  offspring  in 
dihybrid  crosses  will  always  be  sixteen  in  nunil:>er,  or 
that  they  will  always  conform  strictly  to  the  theoreti- 
cal expectation  of  (3  +  1)-.  The  ofispring  ol)tained 
undoubtedly  obey  the  laws  of  chance,  but  the  greater 
the  number  of  offspring,  the  nearer  they  come  to  fall- 
ing into  the  expected  grouping. 

The  sixteen  possible  zygotes  resulting  from  a 
diliybrid  cross  will  give  rise  to  sixteen  possible  kinds 
of  indi\dduals  which  in  turn,  as  will  be  demonstrated 
directly,  present  four  kinds  of  phenotypic  and  nine 
kinds  of  genotypic  constitutions. 

A  dihybrid  mating,  using  the  same  symbols  em- 
ployed in  the  case  just  described,  would  be  expressed 
algebraically  as  follows  :  — 

SG+         WY+         SY+         WG  =  all  the  possible  egg  gametes 
SG+         WY+         SY+  WG  =  all  the  possible  sperm  gametes 

SGSG+    SGWY+    SGSY+    SGWG 

SGWY  +WYWY+    WYSY+    WYWG 

SGSY  +    WYSY  +SYSY+    SYWG 

SGWG +    WYWG +    SYWG+WGWG 

SGSG+2  SGWY+2  SGSY+2  SGWG+WYWY+2  WYSY+  2  WYWG+SYSY+ 2SYWG+  WGIVG 

The  second  and  the  ninth  items  in  this  result  are 
alike ;    by  combining  them  the  revised  result  reads  :  — 

SGSG+4  SGWY+2  SGSY+2  SGWG+WYWY+2  WYSY+2  WYWG+SYSY+WGWG 

There  are  then  these  nine  different  combinations 
of  germinal  characters  or  nine  difTerent  genotypes 
in  any  dihybrid  cross.  By  placing  the  recessive  char- 
acters in  parentheses,  whenever  the  corresponding 
dominant  is  present  to  indicate  that  the   dominant 


136 


GENETICS 


causes  the  former  to  recede  from  view,  these  nine 
genotypes  may  be  combined  into  four  phenotypes  as 
follows :  — 


Phenotypes     .     . 

9SY 

3SG 

3  WY 

\WG 

Genotypes      .     . 

^S(G)(W)Y 

SGSG 

WYWY 

WGWG 

2S(G)Sr 

2SG{W)G 

2  WYW{G) 

2SY{W)Y 

SYSY 

From  this  analysis  it  may  be  said  that  the  Mendelian 
ratio  for  a  typical  dihybrid  is  phenotypically  9:3:3:  1, 
while  that  for  a  monohybrid,  as  we  have  already  seen, 

Cf-  SG  WY  SY  WG 

Q  i  i  4.  I 


s&-» 


WY^ 


SY 


WG 


SG 

SG 

© 

WY 
SG 

© 

SY 

SG 

® 

WG 
SG 

® 

SG 
WY 

® 

WY 
WY 

® 

SY 
WY 

© 

WG 
WY 

® 

5G 
SY 

© 

WY 
SY 

® 

SY 
SY 

® 

WG 
SY 

® 

SG 
WG 

@ 

WY 
WG 

@ 

SY 
WG 

@ 

WG 
WG 

@ 

Fig.  44. — Diagram  to  illustrate  the  possible  combinations  arising  in  the 
second  filial  generation  (F2)  following  a  cross  between  yellow-smooth 
YS  and  green-wrinkled  GW  peas. 

is  3:1.  This  expected  ratio  corresponds  essentially 
with  the  actual  results  Mendel  obtained  in  crossing 
smooth-yellow  and  wrinkled-green  peas. 


SEGREGATION   AND   DOMINANCE 


137 


Figure  44  presents  a  graphic  representation  of  the 
different  combinations  resulting  from  a  dihybrid  cross 
follomng  the  checkerboard  plan  used  in  Figure  42 
to  illustrate  monohybrids. 

The  nine  genotypes  and  four  phenotypes  which 
result  from  a  dihybrid  cross  are  shown  in  the  following 
squares. 


Number  in 
Each  Class 

Genotype 

Number  of  Squares  1    ^ 

in  Fig.  44            ,    Phenotype 

Number  in 
Each  Class 

1 

SYSY 

11 

SY 

2 

{W)YSY        '            7-10 

9 

2 

S(G)SY 

3-9 

4 

S(G)(W)Y 

2-5- 12- 15 

1 

SGSG 

1 

SG 

3 

2 

SG{W)G 

13.  4 

1 

WYWY                        6 

WY 

3 

2 

WYW{G) 

8-14 

1 

WGWG                       16 

WG 

1 

16 

! 

16 

Another  illustration  of  dihybridism  is  shown  in 
Figures  45  and  46  which  is  based  upon  data  fur- 
nished by  the  Davenports.^  In  the  matings  given 
here,  dark  or  pigmented  hair,  represented  by  the  solid 
black  circles,  is  dominant  over  light-colored,  that  is, 
unpigmented  or  slightly  pigmented  hair,  symbolized 
by  the  open  circles,   while  curly  hair  is   dominant 

1  "Heredity  of  Eye-color  in  Man,"  Mcnce.  X.  S.  26,  p.  589,  1907; 
"  Heredity  of  Hair  Form  in  Man,"  Amer.  Nat.  42,  p.  341,  1908.  Daven- 
port, C.  B.  and  G.  C. 


138 


GENETICS 


over  straight,  represented  by  crooked  and  straight 
lines  respectively  in  the  diagram.  In  other  words, 
the  presence  of  pigment  is  dominant  over  the  ab- 
sence of  pigment,  while  the  factor  that  causes  curli- 
ness  is  dominant  over  the  absence  of  this  factor, 
with  respect  to  human  hair. 


HA/f^ 


'  ^ — — r;^    T^' — .  ^/ 


KEY  TO  Symbols 

•  =  Dark 

O  =  Li^ht 

J  =  CurlLj 

—  =  Straight 


'°^s    DARK 

Fig.  45. — The  heredity  of  human  hair  according  to  data  by  C  B.  and 
G.  C.  Davenport.  The  arcs  represent  the  somatoplasms  of  four  indi- 
viduals. Within  the  arcs  are  the  gametes  formed  by  these  individuals. 
The  dominant  character  is  placed  on  the  outside  of  the  arc  where  it 
will  be  visible. 


SEGREGATION  AND  DOMINANCE         139 


When  a  homozygous  individual  with  dark  curly 
hair  crosses  with  a  homozygous  individual  with  light 
straight  hair,  all  the  offspring  have  dark  curly  hair. 

The  dark  curly-haired  individuals  of  this  second 
generation,  however,  are  heterozygous  with  respect 
to  each  of  these  two  hair  characters.  When  any  two 
individuals  having  this  particular  genotypic  compo- 
sition mate,  therefore, 
they  may  produce  any 


Nu.-nber 

in  6AcK 


GENOTrPE 


4 


2 


DdrA  cur/ij 


© 


Ddrk  strdight 


Light  cur/ij 


one  of  four  possible 
phenotypes  — dark 
curly,  dark  straight, 
light  curly  or  light 
straight  haired  individ- 
uals. These  four  phe- 
notypes in  turn  will 
present  nine  different 
genotypic  combina- 
tions out  of  sixteen  pos- 
sible cases,  as  shown  in 
Figure  46. 

Figure  45  further- 
more serves  to  make 
clear,  first,  the  distinc- 
tion between  somato- 
plasm and  germplasm ; 
second,  the  maturation 
of  germ-cells  ;  third,  the 
segregation  of  gametes;  and  fourth,  the  formation  of 
zygotes  in  sexual   reproduction. 

The  cells  of  the  somatoplasm  are  represented  as 


le 


© 


Phenotype: 


Light  strdight 


Nuii<b«r 


;6 


Fig.  46.  —  Diagrams  showing  the  pos- 
sible genotypic  and  phenotypic  com- 
binations resulting  when  two  hetero- 
zygous individuals,  with  dark  curly 
hair,  mate.  Symbols  are  the  same 
as  in  Figure  45. 


140  GENETICS 

making  up  the  arcs  within  which  are  inclosed  the 
germ-cells  after  their  reduction  through  maturation, 
which  results  in  giving  to  each  germ-cell  half  the 
number  of  determiners  that  are  present  in  the  soma- 
tic cells. 

It  will  be  remembered  that  when  two  gametes, 
or  mature  germ-cells,  unite,  they  form  a  zygote 
having  the  proper  number  of  determiners  normal  to 
the  species  in  question  instead  of  double  that  number. 
Symbols  for  dominant  characters  in  the  diagram  are 
placed  on  the  outside  of  the  somatic  arcs,  because 
these  are  the  characters  that  are  visible  or  pheno- 
typic,  while  the  non-apparent  recessives  are  placed 
on  the  inside  out  of  sight. 

11.   The  Case  of  the  Trihybrid 

Mendel  went  even  further  and  computed  the 
possibilities  which  would  result  when  two  parents 
were  crossed  differing  from  each  other  with  respect 
to  three  unit  characters.  He  found  that  the  results 
actually  obtained  by  breeding  closely  approximated 
the  theoretical  expectation. 

This  expectation  in  the  case  of  a  trihybrid  cross  is 
that  the  cross-breds  resulting  will  all  exhibit  the 
three  dominant  characters,  while  their  genotypic 
constitution  will  include  six  factors,  namely,  these 
three  dominant  characters  plus  their  corresponding 
recessives  or  "absences." 

Cross-breds  of  the  first  generation  will,  therefore, 
have  eight  possible  kinds  of  triple  gametes  and  when 
interbred  may  form  a  possible  range  of  sixty-four 


SEGREGATION  AND   DOMINANCE         141 


(8  X  8)  different  zygotes,  which  corresponds  to  a 
monohybrid  raised  to  the  third  power  (3  +  1)^ 
These   sixty-four   zygotes   group   together   in   eight 


RSP 

i 


RsP 

1 


RSb 

i 


Rsp 

■l' 


SP 

i 


rsP 

i 


r  Sp 

I 


r  »p 

I 


?i 

RSP- 
RsP- 
RSp- 
Rsp- 
rSP- 
rsP- 
r  Sp  - 

r  sp  - 

Fig.  47.  —  Diagram  showing  the  possible  combinations  in  a  guinea-pig 
trihybrid  of  the  F2  generation.  R,  rosetted  coat ;  r,  non-rosetted  coat 
(absence  of  R)  ;  S,  short  hair  ;  s,  angora  hair  (absence  of  S)  ;  P,  pig- 
mented ;  p,  albino  (absence  of  pigment).  The  eight  possible  triple 
gametes  of  each  parent  are  placed  in  the  upper  and  left  hand  margins. 
Each  of  the  sixty-four  squares  represents  a  possible  zygote  or  ferti- 
lized egg,  having  received  a  triple  gamete  from  each  parent. 

different    phenotypes    and    twenty-seven    different 
genotypes. 

The  trihybrid  cross  with  its  resulting  combinations 
is  well  illustrated  by  Castle's  work  on  guinea-pigs 
which    confirms    the    Mendelian    hypothesis    on    an 


RSP 
RSP 

RsP 
RSP 

RSp 
RSP 

Rsp 
RSP 

rSP 
RSP 

rsP 
RSP 

rSp 
RSP 

rsp 
RSP 

RSP 
RsP 

RsP 
RsP 

RSp 
RsP 

Rsp 
RsP 

rSP 
RsP 

rsP 
RsP 

rSp 
RsP 

rsp 

RsP 

RSP 
RSp 

RsP 

RSp 

RSp 
RSp 

Rsp 
RSp 

rSP 
RSp 

rsP 

RSp 

rSp 
RSp 

rs  p 
RSp 

RSP 
Rsp 

RsP 
Rsp 

RSp 
Rsp 

Rsp 
Rsp 

rSP 
Rsp 

rsP 
Rsp 

rSp 
Rsp 

rsp 
Rsp 

RSP 

rSP 

RsP 

rSP 

RSp 
rSP 

Rsp 

rSP 

rSP 
rSP 

rsP 
rSP 

rSp 
rSP 

rsp 
rSP 

RSP 

rsP 

RsP 
rsP 

RSp 

rsP 

Rsp 
rsP 

rSP 
rsP 

reP 
rsP 

rSp 
rsP 

rs  p 
rsP 

RSP 

rSp 

RsP 
rSp 

RSp 

rSp 

Rsp 
rSp 

rSP 
rSp 

rsP 
rSp 

rSp 
rSp 

rs  p 
rSp 

RSP 
rsp 

RsP 
rsp 

RSp 
rsp 

Rsp 
rsp 

rSP 
rsp 

rsP 
rsp 

rSp 
rsp 

rs  p 
rsp 

142 


GENETICS 


Number  in 
each  class 


2 


2 


2 


8 


2 


2 


2 


2 


2 


64 


Genotype 


SS  PP  RR 

SS 

pp 

RR 

Ss 

PP  RR 

Ss 

Pp 

RR 

SS  PP  Rr 

SS 

Pp 

Rr 

Ss 

PP  Rr 

Ss 

Pp 

Rr 

SS 

pp 

RR 

Ss 

pp 

RR 

SS 

pp 

Rr 

Ss 

pp 

Rr 

ss 

PP  RR 

ss 

Pp 

RR 

ss 

PP  Rr 

ss 

Pp 

Rr 

ss 

PP 

rr 

SS 

Pp 

rr 

Ss 

PP 

rr 

Ss 

Pp 

rr 

ss 

pp 

RR 

ss 

pp 

Rr 

ss 

pp 

rr 

Ss 

pp 

rr 

ss 

PP 

rr 

ss 

Pp 

rr 

ss 

PP 

rr 

Phenotype 


SPR 

Short,  pigmented,  resetted 


SpR 
Short,  albino,  rosetted 


sPR 

Angora,  pigmented,  rosetted 


SPr 

Short,  pigmented,  non-rosetted 


spR 
Angora,  albino,  rosetted 


Spr 
Short,  albino,  non-rosetted 


sPr 

Angora,  pigmented,  non-rosetted 


spr 
Angora,  albino,  non-rosetted 


Number  in 
each  class 


27j 


9 


9 


3 


64 


SEGREGATION   AND   DOMINANCE        143 

extensive  scale.  In  Figure  47  dominant  characters 
are  represented  by  capital  letters,  while  recessives  or 
absences  are  indicated  by  corresponding  small  letters. 

When  a  smooth,  or  non-rosetted  (r),  short-haired 
(8),  pigmented  (P)  guinea-pig  is  crossed  with  a 
rosetted  (i?),  long-haired  (s),  albino  (p)  guinea-pig, 
all  the  offspring  appear  to  be  of  one  phenotypic 
constitution,  namely,  rosetted,  short-haired,  and 
pigmented  (RSP).  Their  genotypic  constitution  is 
represented  by  the  formula  RrSsPp.  These  six 
factors  may  form  eight  possible  triple  gametes,  as 
follows  :  RSP,  RsP,  RSp,  Rsp,  rSP,  rsP,  rsp.  When 
two  germ-cells  each  made  up  of  these  eight  triple 
gametes  unite  in  sexual  reproduction,  they  will  give 
rise  to  sixty-four  (8  X  8)  possible  zygotes  as  dis- 
played in  Figure  47. 

An  analysis  of  Figure  47  shows  among  the  off- 
spring eight  different  phenotypes  in  the  ratio  of 
27:9:9:9:3:3:3:1  and  27  different  genotypes  in 
the  proportions  indicated  on  the  opposite  page.  The 
order  of  the  three  pairs  of  symbols  is  changed  from 
that  in  Figure  47  to  emphasize  the  fact  that  with 
independent  unit  characters  the  order  is  immaterial. 

12.   Conclusion 

Although  the  ratios  for  more  than  a  trihybrid 
were  computed  by  Mendel,  the  experimental  test 
has  never  been  carried  out,  since  it  involves  such 
large  and  complicated  proportions. 

In  the  case  of  four  differing  unit  characters  in  the 
parental  generation,  the  offspring  of  the  quadruple 


144  GENETICS 

hybrids  derived  from  such  an  ancestry  would  in- 
clude 25Q  or  (3  +  1)  ^  possibilities  instead  of  64  or 
(3  +  1)  ^,  as  in  the  case  of  trihybrids.  When  ten 
differing  characters  are  combined  in  the  parental 
generation,  there  would  result  over  a  million  possible 
kinds  of  offspring  among  the  hybrids  of  the  second 
generation,  (3  +  1)  ^^  =  1,048,576. 

From  the  foregoing  it  is  apparent  that  in  practical 
breeding  the  only  hope  lies  in  dealing  with  not  more 
than  one  or  two  characters  at  a  time.  Since  unit 
characters  usually  behave  independently  of  each 
other,  one  may  breed  for  a  single  character  until  it 
is  segregated  out  in  a  homozygous,  that  is  pure, 
condition,  and  then  in  the  same  way  obtain  a  second 
character,  a  third,  and  so  on. 

Thus  in  a  few  generations  of  properly  directed 
crosses  there  can  be  obtained  combinations  of  char- 
acters united  in  one  strain  that  formerly  were  never 
obtained  at  all  or  were  only  hit  upon  by  the  merest 
chance  at  long  intervals.  Herein  lies  the  scientific 
control  of  heredity  which  the  trinity  of  Mendelian 
principles  :  namely,  independent  unit  characters,  seg- 
regation, and  dominance,  has  placed  in  human  hands. 

13.   Summary 

Three  principles  are  concerned  in  Mendel's  law : 
independent  unit  characters,  dominance,  and  seg- 
regation. 

a.  Independent  Unit  Characters.  An  organism, 
although  acting  together  as  a  physiological  and  mor- 
phological whole,  may  be  regarded  from  the  point 


SEGREGATION   AND   DOMINANCE        145 

of  view  of  heredity  as  consisting  of  a  large  number 
of  independent  heritable  unit  characters. 

b.  Dominance.  In  the  gerniplasm  there  are  cer- 
tain determiners  of  unit  characters  which  dominate 
others  during  the  development  of  the  somatoplasm. 
In  other  words,  they  determine  the  apparent  charac- 
ter of  the  organism  by  causing  that  character  to 
become    visible. 

The  alternative  recessive  characters,  although  they 
may  be  present  in  the  gerniplasm,  are  unable  to  be- 
come manifest  in  the  somatoplasm  so  long  as  the 
dominant  characters  are  present.  When,  however,  a 
dominant  character  is  absent,  its  recessive  alterna- 
tive becomes  manifest. 

c.  Segregation.  Unit  characters,  although  they 
may  be  intimately  associated  together  in  the  indi- 
vidual, during  the  complicated  process  of  maturation 
that  always  precedes  the  formation  of  a  new  indi- 
vidual, separate  or  segregate  out  as  if  independent 
of  each  other  and  thus  are  enabled  to  unite  into 
new  combinations. 


CHAPTER  VIII 

REVERSION    TO    OLD    TYPES    AND    THE    MAKING 

OF   NEW   ONES 

1.  The  Distinction  between  Reversion  and 

Atavism 

There  are  two  ways  in  which  types  of  animals  or 
plants  that  are  different  from  the  present  ones  may 
be  conceived  to  arise,  namely,  by  the  reappearance 
of  old  types  and  by  the  formation  of  new  ones.  In 
the  reappearance  of  old  types  a  distinction  may  be 
drawn  between  reversion  and  what  has  been  termed 
atavism. 

Atavism,  or  "grandparentism,"  may  be  defined  as 
skipping  a  generation  with  the  result  that  a  particu- 
lar character  in  the  offspring  is  unlike  the  corre- 
sponding character  in  either  parent,  but  instead, 
resembles  the  character  in  one  of  the  grandparents. 

In  reversion,  on  the  contrary,  a  character  reappears 
which  has  not  been  manifest  perhaps  for  many  gen- 
erations, although  it  was  actually  present  in  some 
remote  ancestor.  J.  Arthur  Thomson's  definition 
of  reversion  is  :  "All  cases  where  through  inheritance 
there  reappears  in  an  individual  some  character 
which  was  not  expressed  in  his  immediate  lineage, 
but  which  had  occurred  in  a  remoter,  but  not  hypo- 
thetical,  ancestor." 

146 


OLD   TYPES  AND   NEW 


147 


This  distinction  between  atavism  and  reversion 
becomes  clearer  by  illustration. 

If  heterozygous  brown-eyed  individuals  mate, 
there  is  one  possibility  in  four  that  their  offspring 


c^;S^^>9 


Grandmother     grandfather 


.t*oMOr, 


%lu}\^  Duplex 


HorflOZYGOTE        HOMOZYGOTE 

Duplex'j 


/ 

I 


Heterozygote     \  ^ 
'  Simplex ^ 


Mother 

B. 

1    \ 

y         / 
/ 

Meter  o'zYGOTE 

1  \^v^         ^'        Sim'plexX 

I  ■        » ~1  I  >  "^ T |/\f  A  V 1  f"!^    ^-     ■  -^    * 


^02Y&° 


'^t^/liplei^ 


HETfROZYGOTE         HOMOZYGOTE        HETEROZYGOTE 
Simplex  Duplex  5/mplex 

Fig.  48.  —  Three  generations  of  a  Mendeliau  monohj^brid.  The  outlines 
represent  the  somatoplasms  with  the  phenotypio  character  on  the  out- 
side. The  black  symbols  inclosed  within  the  somatoplasm  stand  for 
the  germplasm  in  the  form  of  gametes.  The  short  dotted  arrows  indi- 
cate the  relation  between  germplasm  and  somatoplasm.  The  long 
dotted  arrows  indicate  possible  recombinations  of  germplasms. 


will  have  blue  eyes  unlike  their  own,  but  like  the  two 
blue-eyed  grandparents.  Such  a  blue-eyed  child 
would  be  an  instance  of  atavism.     The  explanation 


148  GENETICS 

of  this  apparently  inconsistent  hereditary  behavior  is 
perfectly  simple  in  the  light  of  the  Mendelian  ratios, 
as  shown  diagrammatically  in  Figure  48,  in  which  the 
circles  represent  the  blue-eyed  and  the  squares  the 
brown-eyed  character. 

This  figure  also  illustrates  what  typically  occurs  in 
the  formation  of  Mendelian  monohybrids  of  the  first 
and  second  filial  generations.  The  squares  are 
symbols  for  the  dominant  characters,  while  the  circles 
are  symbols  for  the  recessive  characters.  When  the 
two  are  superimposed,  the  circle  recedes  from  view, 
The  large  outside  figures  indicate  the  somatoplasm, 
therefore  the  phenotype.  The  small  inclosed  figures 
indicate  the  germplasm,  therefore  the  genotype.  The 
short  dotted  arrows  indicate  what  it  is  that  deter- 
mines the  somatoplasm  in  each  case,  while  the  long 
dotted  arrows  show  what  possible  recombinations  of 
germplasms  can  be  made.  Child  No.  4  is  an  "  ex- 
tracted recessive"  derived  from  dominant  parents, 
but  with  one  recessive  grandparent  on  each  side.  It 
is  a  case  of  "atavism,"  or  taking  after  the  grand- 
parent. Notice  that  atavism  can  occur  only  by 
alternative  inheritance. 

To  quote  Davenport:  *'In  the  majority  of  cases 
atavism  is  a  simple  reappearance  in  one  fourth  of  the 
offspring  of  the  absence  of  a  character  due  to  the 
simplex  nature  of  the  character  in  both  parents." 

An  illustration  of  reversion  would  be  the  reappear- 
ance of  the  ancestral  jungle-fowl  pattern  in  domestic 
poultry  or  of  the  slaty  blue  color  of  the  ancestral 
rock-pigeon  among  buff  and  white  domestic  pigeons, 


OLD   TYPES  AND   NEW  149 

for  the  ancestral  character  or  characters  in  this  type 
of  hereditary  behavior,  as  said  before,  reappear  only 
after  a  lapse  of  many  generations. 

2.   False  Reversion 

"Around  the  term  *  reversion,'  "  Bateson  observes, 
"a  singular  set  of  false  ideas  have  gathered  them- 
selves." In  proof  of  this  statement  there  may  be  cited 
at  least  five  categories  of  apparent  reversion  which 
properly  ought  not  to  be  classed  as  true  reversion. 

a.   Arrested  Developjnent 

Feeble-mindedness  is  not  reversion  to  ancestral 
forms  of  less  intelligence,  but  an  instance  of  arrested 
development  when,  for  some  reason,  the  individual 
fails  to  accomplish  his  normal  cycle  of  development. 

Likewise  harelip  in  man  is  not  a  case  of  reversion 
to  rabbit-like  ancestors  in  which  harelip  is  the  nor- 
mal condition,  but  it  is  ordinarily  due  to  an  arrest  or 
failure  of  certain  embryonic  steps  that  are  essential 
to  the  development  of  the  usual  form  of  human  lip. 

b.    Vestigial  Structures 

These  are  the  vanishing  remains  of  characters  that 
were  formerly  of  significance.  They  do  not  represent 
something  latent  that  is  now  /•^'appearing,  for  they 
have  never  yet  disappeared  phylogenetically,  and  con- 
sequently they  cannot  be  regarded  as  true  reversions. 

The  muscles  under  the  scalp  which  enable  those 
persons  possessing  them  to  wiggle  the  ears ;  the 
palatine  ridges  in  the  roof  of  the  mouth  of  many 


150  GENETICS 

babies  and  some  adults  which  resemble  the  ridges 
in  the  roof  of  a  cat's  mouth ;  the  vermiform  appen- 
dix, a  necessary  part  of  the  digestive  apparatus  of 
many  animals  but  fraught  so  often  with  evil  conse- 
quences to  man ;  these  and  scores  of  similar  charac- 
ters, which,  taken  together,  make  man  in  the  eyes 
of  the  comparative  anatomist  a  veritable  old  curi- 
osity shop  of  ancestral  relics,  are  the  last  traces  of 
characters  which  formerly  had  a  significance  in  some 
of  man's  forbears.  Having  lost  their  usefulness, 
these  structures  still  hang  on  to  the  anatomical 
household  as  pensioners.  They  have  not  been  re- 
called from  the  past,  but  have  always  been  with  us, 
although  of  diminishing  importance.  In  no  sense, 
therefore,  can  they  be  called  reversions. 

c.    Acquired  Characters  resembling  Ancestral  Ones 

Sometimes  the  drunken  descendant  of  a  drunken 
great-grandparent  has  acquired  this  characteristic 
through  his  own  initiative  quite  aside  from  any  an- 
cestral contribution  to  his  germplasm.  This  is  not 
reversion.  It  is  a  reacquisition  which  resembles 
the  ancestral  condition. 

Again,  tame  animals  that  run  wild  acquire  habits 
resembling  those  of  their  wild  ancestors,  but  this  is 
not  necessarily  reversion.  It  is  the  natural  response 
of  feral  animals  to  the  conditions  of  wild  life. 

d.    Convergent  Variation 

The  European  hedgehog,  Erinaceus,  an  insecti- 
vore,  the  American  porcupine,  Erithizon,  a  rodent, 
and    the    Australian    spiny    anteater.     Echidna,    sl 


OLD   TYPES  AND   NEW  1.51 

monotreme,  are  all  mammals  which  have  developed 
in  a  similar  manner  the  very  peculiar  device  of  der- 
mal spines.  There  is  no  reason,  however,  for  regard- 
ing this  character  as  due  to  descent  from  a  common 
spiny  ancestor.  It  is  not  reversion  to  an  ancestral 
type,  but  rather  a  case  of  convergent  variation. 
Similarity  does  not  always  indicate  genetic  continuity. 
In  the  case  of  birds  albinism,  melanism  and  fla- 
vism  are  modifications  of  ordinary  pigmentation  which 
appear  irregularly  among  many  different  species  as 
pathological  "  sports,"  but  no  one  of  these  conditions 
can  be  regarded  as  reversions  to  ancestral  white, 
black,  or  yellow  types. 

e.   Regression 

Galton's  "law  of  regression"  refers  to  the  w^ide- 
spread  phenomenon  already  explained  of  a  constant 
swinging  back  to  mediocrity  which  the  breeder  must 
oppose  with  continual  selection  in  order  to  maintain 
the  standard  of  any  particular  strain.  We  have 
seen  that  within  a  "pure  line,"  regression  is  complete 
and  that  in  populations  made  up  of  a  mixture  of 
pure  lines  it  is  a  factor  always  to  be  reckoned  with. 
Regression,  however,  has  to  do  with  fluctuating  varia- 
tions and  does  not  bring  about  a  permanent  change 
of  type.  It  should,  therefore,  not  be  confused  with 
reversion. 

3.   Explanation  of  Reversion 

Darwin,  who  did  not  always  differentiate  between 
reversion  and  atavism,  suggested  that  reversion  was 


152  GENETICS 

due  sometimes  to  the  action  of  a  more  natural  en- 
vironment, as  in  the  case  of  animals  set  free  after 
having  been  in  captivity,  and  sometimes  to  hybridi- 
zation, since  there  seems  to  be  a  general  tendency  of 
hybridized  organisms  to  "revert"  to  ancestral  types. 
It  is  now  known  that  reversion,  like  atavism,  is 
simply  a  case  of  latent  characters  becoming  apparent 
according  to  the  Mendelian  principle  of  segregation. 
To  quote  Davenport:  "There  is  nothing  more  mys- 
terious about  reversion,  from  the  modern  standpoint, 
than  about  forming  a  word  from  the  proper  com- 
bination of  letters." 


4.   Some  Methods  of  improving  Old  and  estab- 
lishing New  Types 

a.  The  Method  of  Hallet 

This  method,  which  was  formulated  by  the  English 
wheat-grower  Hallet  in  1869,  has  been  in  common 
use  for  a  long  time.  It  consists  in  placing  the  organ- 
isms to  be  bred  in  the  very  best  possible  environment 
and  then  choosing  those  individuals  which  make  the 
best  showing  as  the  stock  from  which  to  breed  further, 
a  procedure  based  upon  the  deep-seated  belief  that 
acquired  characters  are  inherited. 

For  example,  in  a  field  of  wheat,  plants  near  the 
edge  of  the  field  which,  from  lack  of  crowding  or  by 
reason  of  proximity  to  an  extra  local  supply  of  fer- 
tilizer or  any  other  favorable  environmental  factor, 
make  a  more  vigorous  growth  than  their  neighbors. 


OLD   TYPES   AND   NEW  153 

are  selected  in  the  hope  that  the  gains  made  by  them 
will  be  maintained  in  their  offspring. 

We  have  seen  that  it  is  very  questionable  whether 
acquired  characters  which  are  due  to  environmental 
conditions  play  any  role  whatever  in  heredity.  The 
phenotypic  character  does  not  always  indicate  what 
the  germplasm  will  subsequently  do,  and  when  the 
true  genotypic  constitution  of  the  germplasm  is  still 
further  masked  by  the  temporary  fluctuations  caused 
by  a  modified  environment,  it  is  increasingly  difficult 
to  select  wisely  from  the  display  of  variants  those 
which  will  produce  the  best  ancestors  for  the  future 
stock. 

That  this  common  procedure  of  selecting  the  best- 
appearing  animal  in  the  flock  and  the  biggest  ear  of 
corn  in  the  bin,  has  met  with  a  large  degree  of  success 
in  the  past  is  due  entirely  to  the  fact  that  in  many 
instances  the  phenotypic  character  is  an  actual  ex- 
pression of  the  genotypic  constitution.  This  is  not 
always  the  case,  however,  and  we  cannot  now  fail 
to  see  that  the  method  is  blind  and  full  of  error.  Its 
successes  are  due  to  the  indirect  results  of  chance 
rather  than  to  a  direct  control  of  the  factors  of  hered- 
ity. The  great  proportion  of  failures  resulting  from 
this  procedure  now  find  a  reasonable  explanation  from 
the  standpoint  of  Mendelism. 

b.  The  Method  of  Rim  pan 

Contrasted  with  the  Hallet  method  of  augmenting 
acquired  characters  and  then  selecting  the  best  display 
of  them,  is  the  method  of  Rimpau,  who  experimented 


154  GENETICS 

for  two  decades  with  various  grains  and,  finally, 
among  other  results,  produced  the  famous  Schand- 
stedt  barley. 

Rimpau's  method  is  to  sow  grain  under  ordinary 
conditions  with  a  minimum  rather  than  a  maximum 
amount  of  fertilizer  and  then  to  select  individuals, 
neither  from  the  rich  spots  nor  from  the  edges  of 
the  field  where  there  is  little  crowding,  but  from  situa- 
tions where  the  environmental  conditions  are  ordi- 
nary or  even  unfavorable.  Individuals  making  a 
good  showing  under  such  usual,  or  even  adverse, 
conditions  are  worthy  by  nature  rather  than  by  nur- 
ture and  are  consequently  most  desirable  as  progeni- 
tors of  future  stock.  By  this  method  the  attempt  is 
not  to  keep  the  progeny  of  single  individuals  sepa- 
rate, but  to  mass  together  the  best  as  they  appear 
under  ordinary  normal  environment. 

This  again  is  an  indirect  method  of  procedure, 
although  the  character  of  the  germplasm  is  more 
nearly  hit  upon  in  this  way  than  by  Hallet's  method, 
since  the  mask  of  temporary  accessory  modifications 
is  stripped  so  far  as  possible  from  the  somatoplasm, 
and  the  phenotype  made  to  approximate  the  geno- 
typical  constitution. 

c.  The  Method  of  de  Vries 

The  method  of  de  Vries  has  already  been  in  part 
described  in  Chapter  IV.  It  depends  upon  the  pres- 
ervation and  exploitation  of  the  mutations  occurring 
in  nature.  It  recognizes  clearly  the  fact  that  change 
of  type  is  dependent  upon  a  germplasmal   variation 


OLD   TYPES   AND   NEW  155 

which  is  largely,  if  not  entirely,  independent  of  environ- 
mental factors. 

Accordingly,  the  work  of  the  successful  })reeder 
consists  in  simply  taking  what  nature  spontaneously 
furnishes  to  him  rather  than  in  attempting  to  force 
nature  into  producing  something  new.  These  muta- 
tions, when  isolated,  may  become  the  progenitors  of 
desirable  new  lines. 

d.  The  Method  of  Vilmorin 

This  is  an  isolation  method  which  has  been  success- 
fully applied  to  the  sugar-beet  industry.  The  seeds 
from  each  plant  to  be  tested  are  sown  in  separate  beds 
from  which  upon  maturity  samples  are  taken  and 
tested  for  sugar  content.  The  plants  from  the  bed 
furnishing  the  sample  which  contains  the  highest  per- 
centage of  sugar  are  then  used  as  the  seed  producers 
for  the  next  generation.  In  this  way  by  continual 
selection  an  improved  strain  may  be  maintained. 

e.  The  Method  of  Johanssen 

The  method  of  isolating  pure  lines  or  homozygotes 
out  of  a  mixed  population  has  been  considered  in 
Chapter  VT.  As  in  the  method  of  de  Vries  of  isolating 
mutations  so,  too,  in  the  pure  line  method  it  is  recog- 
nized that  the  germplasm  is  the  source  of  initiatory 
changes  and  that  the  technique  of  establishing  new 
types  consists  in  sorting  out  homozygous  strains  of 
this  germplasm. 

The  method  of  Johanssen  is  quite  different  from 
those  of  Hallet  and  Rimpau  in  that  the  ideal  organi- 


156  GENETICS 

zation  is  not  sought  for  among  phenotypes,  but 
among  genotypes.  It  is  not  the  somatoplasm,  but 
the  germplasm  that  is  selected. 

/.   The  Method  of  Burhank 

This  is  a  method  of  greatly  increasing  the  number  of 
variants  by  promiscuous  hybridization  and  then  of 
eliminating  all  except  those  of  a  desired  phenotypic 
combination.  Indirectly  it  depends  upon  the  princi- 
ple of  the  segregation  of  unit  characters  which  makes 
possible  rearrangements  of  these  characters  according 
to  the  laws  of  chance.  The  characters  themselves 
remain  unchanged,  since  nothing  new  is  produced 
by  hybridization  except  new  arrangements  of  existing 
characters. 

The  spectacular  success  of  Luther  Burbank  in 
"creating"  new  plant  forms  is  due  largely  to  his  very 
extensive  hybridizations,  his  skill  in  detecting  among 
the  varying  progeny  the  winning  phenotype  and  his 
ruthless  elimination  of  the  great  majority  of  varia- 
tions that  do  not  quite  fill  his  requirement. 

The  successful  combinations  must  be  propagated 
in  most  instances  asexually  by  grafting,  cuttings, 
bulbs,  etc.,  rather  than  sexually  through  the  medium 
of  seeds,  because  new  genotypes  which  will  breed  true 
are  not  necessarily  isolated  by  this  procedure.  The 
consequence  is  that  Burbank's  method  cannot  be 
utilized  in  animal  breeding  to  any  great  extent  where 
the  maintenance  of  a  desirable  strain  by  asexual  prop- 
agation is  out  of  the  question. 

It  will  be  seen  that  this  method,  like  the  first  two,  is 


OLD   TYPES  AND   NEW  157 

fortuitous  and  to  a  certain  extent  unscientific  in  that 
no  one  can  repeat  the  exact  conditions  of  the  experi- 
ment and  arrive  at  the  same  results.  It  depends  upon 
the  chance  mixing  up  of  a  large  number  of  possibilities 
and  then  in  not  being  distracted  or  blinded  by  the 
good  while  selecting  the  best.  In  the  hands  of  a  skilful 
plant  breeder  with  unlimited  resources  at  his  com- 
mand it  may  result  in  much  practical  achievement,  but 
it  does  not  particularly  illuminate  the  path  of  other 
breeders  who  wish  to  repeat  the  experiment.  It  is 
after  all  a  selection  of  phenotypes  and,  therefore, 
forever  open  to  error,  since  phenotypes  do  not  always 
indicate  what  the  behavior  of  their  constituent  geno- 
types will  be  in  heredity. 

g.  The  Method  of  Mendel 

The  method  of  Mendel,  like  the  foregoing,  depends 
upon  hybridization  with  the  difference  that  the 
desired  combination  is  sought  directly  by  definite 
predetermined  crosses,  according  to  the  expectations 
of  the  Mendelian  ratios,  rather  than  through  the 
random  result  of  fortuitous  combinations.  This 
method  has  been  rendered  possible  by  the  determina- 
tion of  Mendel's  laws  of  dominance,  and  of  the  inde- 
pendence and  segregation  of  unit  characters  which 
give  to  the  experimental  breeder  definite  expectations 
and  a  method  of  procedure. 

If,  upon  hybridization,  the  desired  character  be- 
haves like  a  recessive,  then  all  that  is  necessary  to 
establish  a  pure  stock  exhibiting  the  character  in 
question,  is  to  breed  two  rccessives  together,  because 


158  GENETICS 

recessives  are  always  homozygous  and,  regardless  of 
their  ancestry,  breed  true. 

On  the  other  hand,  if  the  desired  character  proves 
to  be  a  dominant,  then  it  is  necessary  to  determine 
whether  it  is  present  in  a  duplex  or  a  simplex  condi- 
tion; in  other  words,  whether  it  is  homozygous  or 
heterozygous,  for  only  homozygous  organisms  breed 
true.  Establishing  a  strain  consists,  consequently, 
in  making  an  organism  homozygous. 

The  test  to  determine  whether  a  dominant  character 
is  homozygous  or  heterozygous,  that  is,  whether 
it  will  breed  true  or  not,  can  be  made  by  a  single 
cross  according  to  the  procedure  outlined  in  para- 
graph 8  of  Chapter  VII.  If,  upon  crossing  the 
individual  to  be  tested  with  a  recessive,  it  produces 
an  entirely  dominant  progeny,  then  its  germplasm  is 
duplex  for  this  character,  and  it  will  always  reproduce 
the  character  in  either  duplex  or  simplex  condition 
according  to  what  it  may  be  crossed  with.  When 
crossed,  for  instance,  with  another  duplex  dominant 
like  itself,  a  pure  homozygous  strain  of  the  character 
in  question  will  be  perpetuated. 

If,  on  the  contrary,  the  dominant  character  to  be 
tested  proves  to  be  simplex  or  heterozygous,  as  de- 
termined by  the  fact  that,  when  crossed  with  a  re- 
cessive, 50  per  cent  of  the  progeny  are  recessive,  then 
it  requires  more  than  a  single  generation  to  establish  a 
homozygous  dominant  strain. 

In  random  inbreeding  of  diverse  strains  if  the  re- 
cessives are  constantly  eliminated  as  they  appear,  a 
population  is  gradually  obtained  which  is  composed 


OLD   TYPES  AND   NEW  159 

of  an  increasing  number  of  dominants  so  that  after 
only  a  few  generations  the  chances  are  much  reduced 
that  recessives  will  appear,  which  means  the  practical 
purity  of  the  strain. 

5.  The  Factor  Hypothesis 

It  has  been  ascertained  within  the  last  decade 
that  some  characters  require  more  than  a  single  de- 
terminer to  bring  them  to  expression.  The  idea  of 
compound  determiners  for  a  single  character  may  be 
termed  the  factor  hypothesis  of  heredity.  The  con- 
verse is  also  true,  that  certain  single  determiners 
may  control  more  than  one  character.  For  instance, 
the  determiner  for  gray  hair  in  rats  also  produces  a 
lighter  color  on  the  belly. 

Mendel,  whose  experiments  led  him  to  believe  that 
each  character  depends  upon  only  a  single  deter- 
miner for  the  reason  that  he  worked  on  characters 
severally  belonging  to  different  parts  of  the  plant, 
was  apparently  unaware  of  the  existence,  in  certain 
cases  at  least,  of  compound  determiners. 

These  compound  factors  may  be  arranged  in  va- 
rious categories.     For  example,  there  may  be, — 

(1)  A  complementary  factor  which  is  added  to  a 
dissimilar  factor  in  order  that  a  particular  character 
may  appear ; 

(2)  A  supplementary  factor  which  is  added  to  a 
dissimilar  factor  with  the  result  that  a  character  is 
modified  in  some  way ; 

(3)  A    cumulative  factor    which,    when    added    to 


160  GENETICS 

another  similar  factor,  affects  the  degree  of  expression 
that  a  character  is  given ; 

(4)  An  inhibitory  factor,  which  prevents  the  action 
of  some  other  factor,  and  so  on. 

It  will  be  profitable  to  consider  the  factor  hypothe- 
sis in  some  detail,  since  it  helps  to  explain  both  rever- 
sion and  the  formation  of  new  types. 

a.  BatesoYis  Sweet  Peas 

In  the  course  of  numerous  breeding  experiments 
Bateson  obtained  two  strains  of  white  sweet  peas. 
Lathy  r  us,  which,  when  normally  self -fertilized,  each 
bred  true  to  the  white  color.  When  these  two  strains 
were  artificially  crossed,  however,  the  progeny  all 
had  purple  flowers  like  the  wild  ancestral  Sicilian 
type  of  all  cultivated  varieties  of  sweet  peas. 

Here  was  apparently  a  typical  instance  of  reversion, 
but  according  to  the  factor  hypothesis  the  explanation 
is  this.  The  character  of  purple  color  is  dependent 
upon  two  independent  factors  w^hich,  though  sepa- 
rately heritable,  are  both  required  to  produce  it.  Each 
of  these  white  strains  of  sweet  peas  possesses  one  of 
these  factors  which  can  produce  colored  flowers  only 
when  united  with  its  complement,  a  proof  of  which 
appeared  upon  interbreeding  hybrid  purples  from  such 
a  cross.  In  short,  the  color  purple  depends  upon  the 
action  of  two  complementary  factors  which  follow 
the  behavior  of  a  dihybrid.     (See  Chap.  VII,  par.  10.) 

The  gametic  formulse  for  the  two  strains  of  white 
sweet  peas  used  in  this  experiment  are  Cp  and  cP, 
respectively.     C   stands   for   a   color   factor   without 


OLD   TYPES  AND   NEW 


161 


which  no  color  can  appear,  even  lliougli  pigment  for 
color  may  be  present,  and  c  is  the  absence  of  this 
factor,  while  P  represents  a  purple  pigment  factor 
which  only  finds  expression  in  the  somatoplasm  when 


Fig.  49.  —  Diagram  to  illustrate  the  possible  progeny  from  two  hetero- 
zygous purple  sweet  peas  according  to  data  from  Bateson.  C,  color 
factor  (large  circles)  ;  c,  absence  of  C  (small  circles)  ;  P,  pigment  fac- 
tor (large  crosses)  ;  p,  absence  of  P  (small  crosses).  In  the  zygotes 
within  the  checkerboard  squares  the  gametic  symbols  are  superimposed. 

taken  together  with  the  color  factor  C.  The  small 
letter  p  stands  for  the  absence  of  the  purple  pigment 
factor.  It  will  be  seen  that  each  of  the  white  sweet 
peas  whose  formukie  are  given  above  lack  one  of  the 
two   essential   factors    for   purple   color.     When    the 

M 


162  GENETICS 

two  are  crossed,  however,  all  the  progeny  are  purple 
with  the  formula  CcPp. 

These  hybrid  sweet  peas  upon  gametic  segregation 
theoretically  produce  four  kinds  of  gametes,  CP,  Cp, 
cP,  and  cp  which  may  combine  as  any  other  dihybrid 
in  sixteen  different  ways.  In  this  case,  however, 
these  combinations  group  themselves  into  only  two 
phenotypes,  purple  and  white,  as  indicated  in  the 
accompanying  diagram  (Fig.  49)  in  which  C  and  c 
are  represented  by  large  and  small  circles  respec- 
tively, while  P  and  p  are  correspondingly  indicated 
by  large  and  small  crosses.  The  gametic  symbols  are 
superimposed  to  form  the  zygotes. 

The  theoretical  expectation  here  shown  was  closely 
approximated  in  the  actual  results. 

It  may  be  noted  in  passing  that  the  seven  kinds  of 
white  sweet  peas  resulting  from  the  above  cross,  while 
phenotypically  alike,  that  is,  in  the  zygotic  symbols 
of  Figure  49,  lacking  either  the  large  circle  (color)  or 
the  large  cross  (pigment),  belong  to  three  distinct 
genotypes  as  follows  :  — 


1 

2 
3 


Without  the  pigment  factor  (large  cross) 

Without  the  color  factor  (large  circle) 

Without  either  pigment  (large  cross)  or  color  (large  circle) 


Number  of 
Zygote  in 
Figure  49 


6-  8-14 
11 -12   15 

16 


Among  the  purple  peas  are  the  following  four  geno- 
types :  — 


OLD   TYPES   AND   NEW  163 


1 

2 
3 
4 


Duplex  for  both  color  (large  circle)  and  pigment  (large  cross) 
Duplex  for  color  (large  circle)  but  simplex  for  pigment 

(large  cross) 
Simplex  for  color  (large   circle)  but  duplex  for   pigment 

(large  cross) 
Simplex  for  boLh  color  (large  circle)  and  pigment  (large 

cross) 


Number  op 
Zygote  in 
Figure  49 


2-5 
3-9 


4-7-1013 


b.  Castle's  Agouti  Guinea-pigs 

An  illustration  of  a  supplementary  factor  that  acts 
only  in  conjunction  with  some  other  to  bring  about 
a  modification,  is  the  pattern  factor  demonstrated  by 
Castle  in  his  guinea-pigs. 

The  wild  gray,  or  "agouti, "  color  of  the  hair  of  cer- 
tain guinea-pigs  is  due  to  the  fact  that  pigment  is 
distributed  along  the  length  of  each  hair  in  a  definite 
pattern.  The  tip  of  a  single  hair  is  black  followed  by  a 
band  of  yellow,  while  most  of  the  proximal  part  which 
is  more  or  less  concealed  by  overlapping  hairs  is  a 
leaden  color.  The  distribution  of  pigment  in  such  a 
pattern  gives  the  characteristic  gray,  or  agouti  color 
to  the  coat  when  taken  as  a  whole. 

Castle  demonstrated  the  separate  nature  and  be- 
havior of  such  a  pattern  factor  when  he  discovered 
that  it  is  transmitted  independently  of  pigment,  which  is 
necessary  to  bring  it  to  expression.  He  showed  that 
upon  crossing  a  solid  bhick  guinea-pig,  imquestionably 
possessing  pigment  but  no  "pattern,''  with  a  wliite 


164  GENETICS 

albino  guinea-pig  having  no  pigment,  some  of  the 
offspring  "reverted"  to  the  ancestral  agouti,  or 
"pattern"  type,  thus  proving  that  the  pattern  must 
be  carried  in  this  case  by  the  white  or  albino  guinea- 
pig  as  a  factor  independent  of  the  color  which  is 
necessary  for  its  expression. 

c.  Cuenofs  Spotted  Mice 

Another  instance  of  the  interaction  of  supple- 
mentary factors  is  seen  in  the  spotting  of  piebald 
mice.  Cuenot  discovered  that  such  spotting  is  due 
to  the  absence  of  a  uniformity  factor  which  if  present 
causes  color  to  be  uniformly  distributed  over  the 
entire  coat. 

Both  of  these  independent  factors,  spotting  and 
uniformity,  are  real  and  not  imaginary,  since  they  may 
be  separately  transmitted  through  albino  animals  in 
the  same  way  as  the  pattern  factor  mentioned  above, 
notwithstanding  that  in  albinos  both  are  hidden 
through  the  absence  of  pigment,  upon  the  presence  of 
which  their  visibility  depends. 

Whenever  piebald  or  spotted  animals  appear  in  a 
progeny  derived  originally  from  self-colored  stock, 
it  is  evidently  due  to  the  absence  of  such  a  "uni- 
formity" factor  as  has  just  been  described. 

Gal  ton's  theory  of  "particulate  inheritance" 
(page  121)  is  now  satisfactorily  explained  as  true  al- 
ternative inheritance  in  which  the  mosaic  appearance 
is  caused  by  a  Mendelian  determiner,  in  this  instance 
a  spotting  factor  or,  in  other  words,  the  absence  of 
a  factor  for  uniformity. 


OLD   TYPES  AND   NEW  105 

d.    Miss  Durham  s  Intensified  Mice 

Miss  Durham,  in  her  work  with  mice,  has  demon- 
strated an  intensifying  factor,  the  absence  of  wliich 
she  calls  a  diluting  factor.  The  action  of  the  former 
produces,  as  its  name  implies,  intensity  of  color, 
while  that  of  the  latter  serves  to  lessen  the  degree  of 
intensity  in  which  color  appears. 

These  factors  of  intensity  and  diluteness,  it  should 
be  observed,  do  not  in  any  way  correspond  to  the 
duplex  and  simplex  condition  of  a  dominant  color 
character,  either  of  which  would  straightway  appear 
if  crossed  with  an  albino.  The  factors  of  intensity 
and  dilution  of  color  are  of  an  entirely  different 
nature,  as  they  have  been  proven  to  be  indepen- 
dently transmissible  through  albinos  where  a  color 
character  could  not  appear  because  of  the  absence  of 
pigment. 

The  following  illustration  of  this  kind  of  sup- 
plementary factors  taken  from  Miss  Durham's 
experiments  will  serve  to  make  the  case  clear.  The 
symbols  employed  are  :  — 

B  =  black  pigment  which  masks  brown,  or  chocolate. 
h  =  the  absence  of  B,  consequently  chocolate. 
I  =  intensity  factor. 

i  =  dilution  factor  or  absence  of  intensity. 
C  =  a  complementary  color  factor  acting  with  P. 
P  =  a  complementary  pigment  factor  acting  with  C. 
BICP  =  black. 

BiCP  =  blue  or  maltese  (dilute  black). 
hICP  =  chocolate. 
hiCP  =  silver-fawn  (dilute  chocolate). 


166 


GENETICS 


The  crosses  which  were  made  are  represented  in 
the  table  below,  in  which  the  expectation  according 
to  the  Mendelian  dihybrid  ratios  is  given  in  paren- 
theses after  the  actual  results  of  each  cross. 


Black  {BICP)  X  Silver-fawn  {hiCP) 
Blue  {BiCP)  X  Chocolate  {hICP) 
Blue  {BiCP)  X  Silver-fawn  {hiCP) 


Black 
{BICP) 

Blue 
[BiCP) 

Choco- 
late 
{bICP) 

9(9) 

42(45) 

0(0) 

4(3) 

16(15) 

33(36) 

3(3) 
14(15) 

0(0) 

Silver- 
fawn 
{biCP) 

2(1) 

8(5) 

12(12) 


It  will  be  seen  that  the  actual  results,  even  when 
such  small  totals  are  concerned,  approximate  very 
closely  the  expectation  and  are  entirely  consistent. 


e.    Castle  s  Brown-eyed  Yellow  Guinea-pigs 

Recently  Castle  has  shown  that  in  guinea-pigs 
there  is  an  independent  factor  for  extension  of  pig- 
ment distinct  from  the  uniformity  factor  already 
mentioned.  The  absence  of  this  extension  factor 
("restriction  ")  is  manifested  by  a  lack  of  black 
or  brown  pigment  everywhere  except  in  the  eyes 
and  to  a  slight  extent  in  the  skin  of  the  extremities, 
while  the  distribution  of  yellow  is  wholly  unaffected 
by  it. 

That  such  "extension"  and  "restriction"  factors 
really  exist,  is  proven  in  the  following  way  :  — 

When  a  brown  (chocolate)  guinea-pig  is  crossed 
with  an  ordinary  black-eyed  yellow  one,  the  young 
are    all    black    pigmented,    but    by    cross-breeding 


OLD   TYPES  AND   NEW 


167 


these  hybrid  young  four  varieties  are  obtained  in 
the  next  generation,  viz.,  black,  brown,  bhick-eyed 
yellow,  and  brown-eyed  yellow,  the  latter  a  variety 
unknown  before  Castle's  experiment  in  breeding 
was  made. 

For  the  sake  of  clearness  the  formation  of  the 
brown-eyed  yellow  is  shown  below  in  Figure  50. 


cT  Y^ 


V 

Be 


V 

bE 


be 


S^ 


BE 


BE 


Black 


5 

Be|]BE 

Black 


bE|  BE 

Black 


13 

BE 


b  G 


Black 


V 

Be 


^ 


BE 


Be 


Black 


6 

Be   Be 

Bfack-eijed  Yellow 


JO 

bE  B© 

Black 


14 


be 


B 


Black-eHed  Yellow 


\y 


be 


3 


BE 


bE 


Black 


7 

Be    bE 

Black 


I] 

bEllbE 

Chocolate 


4 


BE 


be 


Black 


8 


B 


e 


be 


Black -eHedYellovj 


12 

bE    be 

Chocolate 


15  16 

be     bE    be     be 

Chocolate      Brown-e\jed  Yellow 


Fig.  50.  —  Diagram  to  illustrate  the  origin  of  a  brown-eyed  yellow  guinea- 
pig  from  two  heterozygous  black  parents  based  upon  Castle's  experi- 
ments. The  factor  for  yellow  (F)  is  present  in  every  gamete  and  is 
consequently  duplex  in  every  zygote  but  is  hidden  whenever  the  fac- 
tor B  is  present.  B,  black  pigment  hiding  brown  or  chocolate  ;  h, 
chocolate  (absence  of  B)  ;  E,  extension  of  B  over  the  entire  body 
hiding  Y ;  e,  restriction  of  B  to  eyes  alone  thus  exposing  Y  over  the 
entire  body. 


168  GENETICS 

Symbols 

B  =  black  pigment,  hiding  brown  or  chocolate. 
b  =  absence  of  B,  or  chocolate. 
Y  =  yellow  pigment,  hidden  by  B. 
E  =  extension  of  B  over  entire  body,  hiding  Y. 
e  =  restriction  of  B  to  eyes  alone,  thus  exposing  Y  over 

the  entire  body. 
C  =  complementary  color  factor  acting  with  P  to  produce 

color. 
P  =  complementary   pigment   factor   acting   with    C   to 
produce  color. 
(The  factors  C  and  P  may  be  omitted  for  the  sake  of 
simplicity,  since  they  are  present  in  each  instance.) 

First  Cross 

"Extended"  chocolate  (bEY)  X  black-eyed  yellow 
(BeY)  =  black   {BbEeYY). 

Second  Cross 

When  these  cross-breds  are  mated  with  each  other, 
they  each  form  four  kinds  of  gametes,  BEY,  BeY, 
bEY,  and  beY,  which  unite  into  sixteen  theoretical 
genotypic  possibilities,  shown  in  Figure  50.  These 
fall  into  four  phenotypes,  nine  black  (BEY),  three 
black-eyed  yellow  (BeY),  three  chocolate  (bEY), 
and  one  brown-eyed  yellow  (beY).  The  actual 
results  in  Castle's  experiments  gave  all  four  kinds 
in  close  numerical  agreement  with  this  expectation. 
The  action  of  extension  and  restriction  factors  is, 
therefore,  plainly  a  case  of  Mendelian  dihybridism 
in  which  two  independent  pairs  of  alternative  char- 
acters are  concerned. 


OLD   TYPES  AND   NEW 


1G9 


6.    Rabbit  Phenotypes 

Perhaps  no  better  application  of  the  factor  hy- 
pothesis may  be  found  than  the  case  of  the  color 
of  rabbits. 

There  are  many  varieties  of  rabbits  so  far  as  color 
is  concerned,  particularly  among  domesticated  races. 
These  varieties  are  now  quite  explainable  by  the 
factor  hypothesis,  as  indicated  in  the  table  below. 
The  sixteen  kinds  of  rabbits  there  catalogued  have 

The  Factor  Hypothesis  applied  to  Colors  of  Rabbits 


Constant 
Factors 

Alternative 
Factors 

Gametic 
Formula 

Phenotypic  Character 

when  Crossitd  with  the 

Same  Kind  of 

G\METir  CoMniN\Tinv 

1 

2 

3 

4 
C 

5 
E 

6 

I 

i 

7 

8 

B 

Y 

U 
u 

A 

AUIEClYBBr] 

Gray 

a 
A 
a 

aUIEC  [YBBr] 

Black 

AuIEC  [YBBr] 

Gray  spotted 

auIEC    [YBBr] 

Black  spotted 

U 

A 

AUiEC  [YBBr] 

Blue-gray 

a 

aUiEC    [YBBr] 

Blue  (Maltese) 

u 
U 

A 
a 

A 

AuiEC    [YBBr] 

Blue-gray  spotted 

Br 

auiEC     [YBBr] 

Blue  spotted 

e 

I 

i 

AUIeC  [YBBr] 

(  Yellow  (with  white  belly 
(      and  tail) 

a 

aUIeC    [YBBr] 

(  Sooty  yellow  (with  yellow 
i      belly  and  tail) 

u 

A 
a 

AuIeC    [YBBr] 

Yellow  spotted 

auIeC     [YBBr] 

Sooty  yellow  spotted 

U 

A 

AUieC    [YBBr] 

Cream 

a 

aUieC     [YBBr]  \ 

Pale  sooty  yellow 

u 

A 
a 

AuieC     [YBBr] 

Cream  spotted 

auieC      [YBBr] 

Pale  sooty  yellow  spotted 

170  GENETICS 

been  obtained  by  Castle  and  other  experimental 
breeders  as  well  as  many  of  the  albino  types  that 
would  double  this  list  if  c,  or  the  factor  for  absence  of 
color,  should  be  substituted  for  C,  the  presence  of 
color,  in  column  4  of  the  table  on  page  169. 

Explanation  of  Symbols  in  the  Foregoing  Table 

Br  =  a  factor  acting  on  C  to  produce  hroivn  pigmentation. 

B  =  a  factor  acting  on  C  to  produce  black  pigmentation. 

F  =  a  factor  acting  on  C  to  produce  yellow  pigmentation. 
The  three  factors,  F,  B,  Br,  are  present  in  every 
rabbit  gamete  and  up  to  date  have  not  been  sepa- 
rable as  independent  unit  characters,  although  they 
have  been  separated  out  in  guinea-pigs  and  mice. 
There  are  no  brown  rabbits,  because  black  always 
goes  linked  with  brown  covering  the  brown  factor. 
Yellow  rabbits  result,  as  explained  below,  through 
the  action  of  factor  e. 

C  =  a  common  color  factor  necessary  for  the  production 
of  any  pigment.  It  was  discovered  in  1903  by 
Cuenot. 
c  =  the  absence  of  C  which  results  in  albinos,  regardless 
of  whatever  pigment  factors  may  be  present. 
By  changing  C  to  c,  sixteen  kinds  of  albinos  would 
be  added  to  this  catalogue,  an  addition  of  one 
phenotype  and  sixteen  genotypes,  all  looking  alike 
but  breeding  differently. 

E  =  Si  factor  governing  the  extension  of  black  and  brown 

pigment,  but  not  of  yellow, 
e  =  the  absence  of  extension  or  restriction  of  black  and 
brown  pigment  to  the  eyes  and  the  skin  of  the 
extremities  only,  while   yellow  remains  extended 
and  visible.     Demonstrated  by  Castle  in  1909. 


OLD   TYPES   AND   NEW  171 

/  =  an  intensity  factor  which  determines  the  degree  of 
pigmentation.  It  can  be  transmitted  indepen- 
dently of  C  through  an  albino.  Discovered  by 
Bateson  and  Durham  in  1906. 

i  =  the  absence  of  intensity  or  dilution.  Dilute  black  = 
blue.  Dilute  yellow  =  cream.  Dilute  gray  = 
blue-gray. 

U  =  SL  factor  for  uniformity  of  pigmentation  or  "  self- 
color"  discovered  by  Cuenot  in  1904. 

u  =  the  absence  of  uniformity  which  results  in  spotting 

with  white. 
A  =  Si  pattern  factor  for  agouti,  or  wild  gray  color,  which 
causes  the  brown  and  black  pigments  to  be  ex- 
cluded from  certain  portions  of  each  hair,  resulting 
in  the  gray  coat.  When  present  in  the  rabbit,  it 
is  also  associated  with  white  or  lighter  color  on 
the  under  surfaces  of  the  tail  and  belly.  It  was 
demonstrated  bv  Castle  in  1907. 

a  =  the  absence  of  the  agouti  or  pattern  factor. 

7.   The  Kinds  of  Gray  Rabbits 

Each  of  the  apparent  kinds  of  gray  rabbits  indicated 
in  the  foregoing  table  may  be  made  up  of  various 
genotypes.  For  instance,  there  are  thirty-two  differ- 
ent genotypes,  each  of  which  is  phenotypically  a  gray 
rabbit.  The  zygotic  formula  for  each  of  these  thirty- 
two  possibilities  is  displayed  in  the  next  table,  and  it 
will  be  seen  that  these  range  all  the  way  from  rabbits 
homozygous  in  all  their  variable  characters  (No.  1) 
to  those  homozygous  in  none  (No.  3'2). 

The  progeny  of  these  various  types  of  gray  rabbits 
when  inbred  wall  consequently  vary  from  the  pure 


17^ 


GENETICS 


The  Kinds  of  Gray  Rabbits  (Color  only) 


Num- 

ber of 

Genotype 
Zygotic  Formula 

Het- 

EROZY- 
GOTIC 

Phenotypes 
When  inbred,  these  kinds  are  produced 

Fac- 

tors 

1 

AAUUIIEECCl 

YBBr][YBBr] 

None 

X 

2 

AAUUIIEECc  [ 

YBBr][YBBr] 

One 

X 

X 

3 

AAUUIIEeCC    [ 

YBBr][YBBr] 

One 

X 

X 

4 

AAUUIiEECC  [ 

YBBr][YBBr] 

One 

X 

X 

5 

AAUuIIEECC  ! 

YBBr][YBBr] 

One 

X 

X 

6 

AaUUIIEECC  [ 

YBBr][YBBr] 

One 

X 

X 

7 

AAUUIIEeCc 

YBBr][YBBr] 

Two 

X 

X 

X 

8 

AAUUIiEECc 

YBBr][YBBr] 

Two 

X 

X 

X 

9 
10 

AAUuIIEECc 
AaUUIIEECc 

YBBr]  [YBBr] 
YBBr][YBBr] 

Two 
Two 

X 
X 

X 

X 

X 
X 

11 
12 
13 
14 

AAUUIiEeCC 
AAUuIIEeCC 
AaUUIIEeCC 
AAUuIiEECC 

YBBr]  [YBBr] 
YBBr]  [YBBr] 
YBBr]  [YBBr] 
YBBr]  [YBBr] 

Two 
Two 
Two 
Two 

X 
X 
X 
X 

X 

X 
X 

X 
X 

X 

X 
X 
X 

X 

X 

X 

15 
16 

AaUUIiEECC 
AaUuIIEECC 

YBBr]  [YBBr] 
YBBr]  [YBBr] 

Two 
Two 

X 
X 

X 
X 

X 

X 

X 

X 

17 

AaUuIiEECC 

YBBr]  [YBBr] 

Three 

X 

X 

X 

X 

X 

X 

X 

X 

18 

AaUuIIEeCC 

[YBBr]  [YBBr] 

Three 

X 

X 

X 

X 

X 

X 

X 

X 

19 

AaUuITEECc 

YBBr]  [YBBr] 

Three 

X 

X 

X 

X 

X 

20 

AaUUIiEeCC 

YBBr]  [YBBr] 

Three 

X 

X 

X 

X 

X 

X 

X 

X 

21 

AaUUIiEECc 

YBBr]  [YBBr] 

Three 

X 

X 

X 

X 

X 

22 

AaUUIIEeCc 

YBBr]  [YBBr] 

Three 

X 

X 

X 

X 

X 

23 

AAUuIiEeCC 

YBBr]  [YBBr] 

Three 

X 

X 

X 

X 

X 

X 

X 

X 

24 

AAUuIIEeCc 

YBBr]  [YBBr] 

Three 

X 

X 

X 

X 

X 

25 

AAUuIiEECc 

YBBr]  [YBBr] 

Three 

X 

X 

X 

X 

X 

26 

AAUUHEeCc 

YBBr]  [YBBr] 

Three 

X 

X 

X 

X 

X 

27 

AAUuIiEeCc 

[YBBr]  [YBBr] 

Four 

X 

X 

X 

X 

X 

X 

X 

X 

X 

28 

AaUUIiEeCc 

[YBBr]  [YBBr] 

Four 

X 

X 

X 

X 

X 

X 

X 

X 

X 

29 

AaUuIIEeCc 

YBBr]  [YBBr] 

Four 

X 

X 

X 

X 

X 

X 

X 

X 

X 

30 

AaUuIiEECc 

[YBBr]  [YBBr] 

Four 

X 

X 

X 

X 

X 

X 

X 

X 

X 

31 

AaUuHEeCC 

[YBBr]  [YBBr] 

Four 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

32 

AaUuIiEeCc 

[YBBr]  [YBBr] 

Five 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

k; 

2- 

CO 
t3 

o 

X 

o 
v; 

CO 

o 

e+ 

a> 

X 

o 

p 
3 

X 

71 

X 

o 

►1 

p 
3 

w 

■o 
o 

<n 

a- 

X 

o 
*<; 

w 

n> 
& 

X 

'i 

> 

o 

•-< 

o 

p 
'< 

CO 

o 

m 

p" 
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m 

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w 

c 

CD 

IK 

1-1 

p 

o 

c 
a- 

a> 

CO 

o 
o 

OLD   TYPES   AND  NEW  173 

gray,  as  in  No.  1,  to  a  gray  from  which  sixteen  pos- 
sible types  of  young  may  be  expected  as  in  No.  32. 
Up  to  the  time  when  Castle's  paper  upon  the  factor 
hypothesis  ^  was  published  in  1909,  nine  genotypic 
kinds  of  gray  rabbits  had  been  obtained  in  his  ex- 
periments, whose  genotypic  formulae  correspond  to 
the  following  numbers  in  the  list :  1,  3,  6,  10,  13,  20, 
22,  28,  29. 

8.  Conclusion 

That  a  relatively  small  number  of  factors  may  pro- 
duce an  extensive  array  of  combinations  is  evident 
from  this  data. 

The  analysis  of  germplasm  by  the  factor  hypothesis 
is  now  being  generally  applied  by  geneticists  to  the 
particular  organisms  with  which  they  are  concerned. 
It  has  been  carried  out  notably  in  detail  by  both 
Bateson  and  Davenport  for  poultry  and  byBaur  for 
the  snapdragon.  Antirrhinum. 

Finally,  the  elucidation  of  the  factor  hypothesis 
makes  any  further  explanation  of  reversion  super- 
fluous. It  is  now  easy  to  see  how  a  particular  char- 
acter may  remain  latent  for  generations  and  at  last 
come  to  expression  only  when  the  missing  factor 
necessary  to  its  activity  is  supplied  by  some  cross. 

It  is  also  clear  how  hybridization,  in  w^hich  many 
characters  are  concerned,  is  bound  to  furnish  far  more 
new  combinations  than  would,  at  first  thought,  be 
expected. 

^  "Studies  of  Inheritance  in  Rabbits."  Carnegie  Institution  Publica- 
tions, No.  114,  1909.  W.  E.  Castle  in  collaboration  \vith  Walter,  Mul- 
lenix  and  Cobb. 


CHAPTER  IX 

BLENDING   INHERITANCE 

1.  Relative  Value  of  Dominance  and  Segre- 
gation 

Of  the  three  fundamental  principles  which  underlie 
"Mendel's  law,"  namely,  segregation,  independence  of 
unit  characters,  and  dominance,  the  principle  of 
dominance  has  been  found  to  hold  true  in  a  surpris- 
ing number  of  cases  and  in  relation  to  very  diverse 
organisms,  notwithstanding  the  fact  that  the  time 
spent  in  the  investigation  of  dominance,  as  that  term 
is  now  understood,  has  been  comparatively  short. 
Doubtless  future  experimentation  will  demonstrate 
the  existence  of  dominance  to  a  far  greater  extent 
than  has  at  present  been  discovered. 

Its  universal  application  is  by  no  means  assured, 
however,  since  the  mathematical  precision  with  which 
it  works,  that  following  its  discovery  in  1900  has  so 
captivated  the  biological  world,  is  beginning  to  give 
way  in  the  face  of  many  exceptions  which  have  been 
steadily  accumulating. 

Even  Mendel  himself  noted  certain  exceptions  to 
the  law  of  dominance,  and  his  followers  have  pointed 
out  with  increasing  emphasis  that  it  is  subject  to 
many  modifications.     It   is   now  understood,  indeed, 

174 


BLENDING   INHERITANCE  175 

that  segregation,  not  dominance,  is  the  most  essential 
factor  in  the  Mendehan  scheme. 

2.   Imperfect  Dominance 

It  frequently  occurs  that  dominance  is  so  imperfect 
that  a  heterozygous,  or  simplex,  dominant  may  be 
distinguished  at  once  by  simple  inspection  from  a 
homozygous,  or  duplex,  dominant,  whereas  the  test 
of  crossing  with  a  recessive  is  necessary  wlienever 
dominance  is  complete,  as  has  been  previously  ex- 
plained. The  single  dose  of  the  determiner  in  such  a 
case  has  plainly,  then,  less  phenotypic  effect  than  a 
double  dose. 

There  are  many  cases  of  imperfect  dominance  among 
flowering  plants.  Cor  reus  has  shown  that  when 
plants  of  a  white-flowering  race  of  the  "four-o'clock," 
Mirahilis  jalapa,  are  crossed  with  those  of  a  red- 
flowering  race,  all  the  offspring  in  the  first  filial  genera- 
tion, unlike  either  parent,  exhibit  rose-colored  flowers. 
When,  however,  these  rose-colored  flowers  are  crossed 
with  each  other,  they  produce  red,  rose,  and  white 
in  the  Mendelian  ratio  of  1 :  2  : 1 ;  that  is,  three  colored 
to  one  white.  The  red-flowering  race  thus  proves 
to  be  homozygous  and  the  rose-flowering  race  hetero- 
zygous. Here  color  dominates  the  absence  of  color, 
or  white,  but  the  degree  of  the  color  depends  upon 
whether  the  dose  of  pigment  is  duplex  or  simplex. 

A  classic  illustration  of  imperfect  dominance  among 
animals  is  the  "blue  Andalusian  fowl,"  the  hereditary 
behavior  of  which  is  illustrated  below  (Fig.  51).  It 
will  be  seen  that  when  two  blue  Andalusian  fo\Als, 


176 


GENETICS 


characterized  by  a  mottled  plumage,  are  bred  together, 
they  produce  three  kinds  of  offspring  in  the  ratio  of 
1:2:1.  Twenty-five  per  cent  are  clear  black,  50  per 
cent  are  blue  Andalusian,  and  25  per  cent  are  white 
"splashed"  with  black.  Both  the  black  and  the 
splashed  white  fow^ls  from  this  cross  prove,  upon 
further  breeding,  to  be  homozygous,  while  the  blue 
Andalusian  itself  is  heterozygous  and  can,  therefore. 


Andalusian 


Andalusian 


i~" 

Black 


T 


T 


1 


Andalusian 


Anda 


T 


T 


T 


usian        Splashed  White 


T 


BlacK         Black     Andalusian         Andalusian     Sp<- White     SpL  White 


Andalusian 

Fig.  51. — The  heredity  of  the  blue  Andalusian  fowl,  an  illustration  of 

"imperfect  dominance." 

never  be  made  to  breed  true.  In  order  to  produce 
100  per  cent  of  blue  Andalusian  chicks,  it  is  necessary 
simply  to  cross  a  splashed  white  with  a  black  Anda- 
lusian. 

There  is  nothing  in  this  case  to  indicate  whether  the 
black  or  the  splashed  white  should  be  regarded  as 
the  homozygous  dominant,  since  dominance  is  im- 
perfect. In  either  case  the  heterozygous  blue  Anda- 
lusian is  at  once  evident  in  the  first  filial  generation 
without  further  crossing. 

A  similar  case  of  imperfect  dominance  is  furnished 
by  the  roan  color  of  cattle  w^hich  results  when  red 
and  white  are  crossed.     If  two  roans  are  mated,  they 


BLENDING   INHERITANCE  177 

produce  red,  roan,  and  white  offspring  in  the  propor- 
tion of  1:2:1,  thus  showing  that  roan  is  a  heterozy- 
gous character  in  which  the  dominance  of  red  is 
imperfect. 

Even  in  cases  of  apparently  perfect  dominance  it 
is  sometimes  possible  by  close  inspection  to  detect 
differences  between  a  pure  dominant  {DD),  Figure  43, 
and  a  heterozygous  dominant  [DR]  when  a  superficial 
examination  is  not  sufficient  to  distinguish  them. 

For  instance,  in  the  cross  between  smooth  and 
wrinkled  peas,  a  microscopic  examination  of  the 
starch-grains  in  the  cotyledons  of  the  hybrid  peas 
shows  that  they  are  of  two  kinds.  Darbyshire  calls 
attention  to  the  fact  that,  in  the  power  of  absorption, 
hybrid  smooth  peas  {DR)  are  intermediate  between 
their  pure  dominant  smooth  {DD)  and  pure  recessive 
wrinkled  {RR)  parents. 

3.   Delayed  Dominance 

A  character  which  is  really  dominant  is  sometimes 
so  late  in  manifesting  itself  in  the  individual  growth 
of  the  offspring  that  it  may  properly  be  termed  a 
delayed  dominant. 

Dark-haired  individuals  often  do  not  acquire  their 
definitive  hair  color  until  adult  life,  and  it  is  common 
knowledge  that  the  eyes  of  an  infant  for  a  consider- 
able period  provoke  no  little  speculation  among  ador- 
ing relatives  as  to  "  whose  eyes  "  they  are. 

According  to  Davenport,  when  a  white  Leghorn 
fowl  is  crossed  with  a  black  Leghorn,  white  being 
dominant  in  this  case,  chicks  are  produced  that  are 

N 


178  GENETICS 

white  with  black  flecks  in  their  plumage.  These  black 
flecks,  however,  disappear  at  the  time  of  the  first 
molt.  The  complete  dominance  of  white  is,  there- 
fore, simply  delayed. 

4.  "Reversed"  Dominance 

In  certain  instances  there  seems  to  be  a  reversal 
of  dominance,  as  may  be  illustrated  by  Lang's  results 
w^ith  snails  (Helix) .  He  has  proven  in  his  experiments 
that  red  snails  are  generally  dominant  over  yellow 
snails,  although  in  certain  cases  there  is  apparently 
an  exception  to  the  rule,  for  snails  with  yellow  shells 
dominate  those  with  red  shells. 

Davenport  also  has  shown  that  although  extra 
toes  are  usually  dominant  over  the  normal  number 
in  poultry,  yet,  in  something  like  20  per  cent  of  the 
cases,  the  normal  number  is  dominant. 

To  speak  of  these  cases  as  instances  of  "reversed 
dominance,"  is  open  to  serious  objection,  since  such 
an  explanation  does  not  agree  with  the  generally 
accepted  "presence  and  absence"  idea  of  heritable 
characters.  It  is  difficult  to  see  how  the  presence  of 
a  certain  determiner  can  dominate  in  a  part  of  the 
offspring  of  any  cross  and  the  absence  of  the  same 
determiner  be  able  to  dominate  the  remainder. 

It  is  perhaps  nearer  the  truth  to  conceive  that  in 
cases  of  apparent  "reversal"  of  dominance  there  is 
an  insufficient  amount  of  a  particular  determiner 
available  to  bring  the  character  concerned  into 
expression.  In  other  words,  although  a  dominant 
character  may  be  present  in  two  cases,  yet  in  one 


BLENDING   INHERITANCE  179 

it  fails,  for  some  reason,  to  become  effective.  This 
interpretation  agrees  with  the  facts  brought  out  by 
subsequent  breeding  in  cases  of  this  sort. 

It  sometimes  occurs  that  a  character  wliich  is 
dominant  in  one  species  may  be  recessive  in  anotlier. 
Horns  are  dominant  in  sheep,  but  recessive  in  cattle. 
White  color  is  recessive  in  rodents  and  sheep,  but 
dominant  in  most  poultry  and  in  pigs. 

5.   Potency 

Davenport  seeks  to  explain  modifications  in  typical 
dominance  as  variations  in  the  potency  of  determiners. 
He  defines  potency  as  follows:  "The  potency  of  a 
character  may  be  defined  as  the  capacity  of  its  germi- 
nal determiner  to  complete  its  entire  ontogeny." 

That  is,  if  the  potency  of  a  determiner,  for  some 
reason,  is  insufficient,  there  may  be  either  an  incom- 
plete or  delayed  manifestation  of  the  character  in 
question,  or  it  may  fail  entirely  to  develop. 

The  variations  of  potency  may  be  grouped  into 
three  general  categories  according  to  the  degree  of 
their  manifestation ;  namely,  total  potency,  partial 
potency,  and  failure  of  potency. 

A  further  word  of  explanation  for  each  of  these 
three  kinds  of  potency  seems  desirable  at  this  point. 

a.   Total  Potency 

This  is  complete  Mendelian  dominance  in  which 
even  the  heterozygotes  produced  by  a  sini])lex  dose 
of  a  character  are  indistinguishable  phenotypically, 
that  is,  by  inspection,  from  the  homozygotes  produced 


180  GENETICS 

by  a  duplex  dose  of  the  same  character.  It  is  as  if 
a  single  bottle  of  black  ink  poured  into  a  jar  of  water 
was  just  as  effective  as  two  bottles  of  ink,  in  forming 
an  opaque  fluid. 

h.  Partial  Potency 

Partial  potency  covers  all  cases  of  incomplete 
dominance,  such  as  those  of  the  four-o'clock  {Mirahilis) 
and  blue  Andalusian  fowls,  where  a  simplex  dose  of 
a  determiner  does  not  produce  the  same  visible  effect 
as  a  double  dose. 

The  dominant  prickly  Jamestown  weed  {Datura)  ^ 
when  crossed  with  a  recessive  glabrous  variety  of 
the  same  plant,  produces  cross-breds  in  the  first 
generation  which  show  only  a  few  prickles  (Bateson) 
(Baur),  following  the  law  of  partial  potency. 

Banded  and  uniformly  colored  snails  also,  when 
crossed  together,  produce  snails  with  shells  showing 
only  a  pale  banding  (Lang). 

Numerous  further  instances  of  incomplete  domi- 
nance could  be  cited. 

c.  Failure  of  Potency 

If  for  any  reason  a  determiner  fails  to  accom- 
plish its  possibilities  in  whole  or  in  part,  then  the 
character  in  question  may  never  become  evident,  and 
the  result,  so  far  as  appearances  go,  is  the  same  as 
if  it  was  a  recessive  lacking  the  determiner  entirely. 

That  the  failure  of  potency  is  not  identical  with 
the  absence  of  a  determiner  can  usually  be  demon- 
strated by  further  breeding,  because  dominants  failing 


BLENDING   INHERITANCE  181 

in  potency,  which  are  either  of  the  formula  DD  or  /)/?, 
may,  if  bred  inter  se,  give  a  various  prof:^eny  among 
which  the  dominant  character  D  is  hkely  to  again 
become  manifest,  while  recessives,  of  the  formula 
RR,  on  the  contrary,  will  always  give  offspring  which 
all  agree  in  the  entire  absence  of  the  character  in 
question. 

Davenport  cites  an  extreme  case  of  failure  of  potency 
in  one  of  two  rumpless  cocks  from  the  same  blood. 
The  character  of  rumplessness  is  due  to  an  inhibitor 
of  tail  development.  That  these  two  cocks  both 
possessed  this  character  was  demonstrated  by  the 
entire  absence  of  any  tail  in  either  case.  The  in- 
hibiting determiner  for  tail  growth  was  so  weak  in 
cock  No.  117,  however,  that,  to  quote  Davenport's 
exact  words  :  "In  the  heterozygote  the  development  of 
the  tail  is  not  interfered  with  at  all,  and  even  in  ex- 
tracted dominants  it  interfered  little  w^ith  tail  develop- 
ment, so  that  it  makes  itself  felt  only  in  the  reduced 
size  of  the  uropygium  and  in-bent  or  shortened  back. 
But  in  No.  116  the  inhibiting  determiner  is  strong. 
It  develops  fully  in  about  47  per  cent  of  all  the 
heterozygotes  and  in  extracted  dominants  may  pro- 
duce a  family  in  all  of  which  the  tail's  development 
is  inhibited." 

Here  were  two  birds  of  the  same  blood,  pheno- 
typically  alike  and  presumably  genotypically  alike, 
which  because  of  an  individual  difference  in  the 
potency  of  the  determiner  for  rumplessness  produced 
quite  different  results  in  their  offspring  although  bred 
to  precisely  the  same  array  of  hens. 


182  GENETICS 

6.  Blending  Inheritance 

In  the  instances  of  imperfect  dominance  given 
above,  where  the  progeny  of  unhke  parents  present 
an  intermediate  condition,  it  is  found  that,  upon 
cross-breeding  these  offspring,  segregation  into  the 
grandparental  types  occurs  just  as  truly  as  in  instances 
of  complete  dominance. 

In  poultry,  for  example,  when  Cochins,  which  are 
"booted,"  and  Leghorns,  which  are  clean-shanked, 
are  crossed,  booting  of  an  intermediate  grade  of  four 
results,  on  a  scale  in  which  ten  represents  complete 
booting,  and  zero  no  booting  or  clean  shank  (Daven- 
port). The  character  of  booting  and  its  alternative 
absence,  however,  segregate  out  in  true  Mendelian 
fashion  when  these  hybrids  are  subsequently  crossed 
together.  It  is  evident  that  dominance  plays  only 
a  secondary  role  in  such  cases,  and  that  the  all-im- 
portant factor  is  segregation. 

Are  there,  then,  any  cases  where  true  fusion  of  hered- 
itary parental  traits  occurs,  in  other  words,  where 
segregation  in  the  second  filial  generation  does  not 
appear?  Does  the  "melting-pot  of  cross-breeding" 
ever  "melt"  the  characters  thrown  into  it  .^ 

It  was  formerly  believed  that  diverse  parents 
generally  produce  intermediate  offspring,  and  that 
this  intermediate  condition  continues  without  any 
segregation  at  all  in  the  form  of  "blending  inheritance," 
but  within  the  last  decade  apparent  cases  of  blending 
inheritance  have  been  thrown  out  of  court  one  after 
the  other  by  the  Mendelians.    Bateson,  in  an  inaugural 


BLENDING    INHERITANCE  183 

address  at  Cambridge  University  in  1908,  stated  that 
what  was  once  beheved  to  be  the  rule  has  now  be- 
come the  exception.  He  goes  on  to  say:  *'One  clear 
exception  I  may  mention.  Castle  finds  that  in  a  cross 
between  the  long-eared  lop  rabbit  and  a  short-eared 
breed,  ears  of  intermediate  length  are  produced ;  and 
that  these  intermediates  breed  approximately  true." 

Let  us  examine  this  "one  clear  exception"  a  little 
more  closely. 

7.  The  Case  of  Rabbit  Ears 

As  a  typical  example  of  blending  inheritance  in 
rabbit  ears  may  be  cited  the  following  case :  — 

A  female  Belgian  hareAvith  an  ear-length  of  118  mm. 
was  crossed  with  a  male  lop-eared  rabbit  with  an 
ear-length  of  210  mm.  The  average  of  these  ear- 
lengths  is  164  mm.  Five  offspring  from  this  pair 
had  ear-lengths,  when  adult,  approximating  this  aver- 
age as  follows:  170,  170,  166,  156,  170,  of  which 
two  were  females  and  three  were  males.  When  from 
this  litter  one  of  the  females  measuring  170  mm.  in 
ear-length  was  subsequently  crossed  with  her  brother 
having  an  ear-length  of  166  mm.,  two  litters  were 
produced  in  which  the  individuals  when  adult  at- 
tained ear-lengths  of  170,  166,  168,  160,  172,  and 
168  mm.  These  results  are  represented  diagram- 
matically  in  Figure  52. 

This  illustration  is  typical  of  many  other  breed- 
ing experiments  made  by  the   same   investigators  ^ 

*  Castle,  in  collaboration  with  Walter,  Mullenix  and  Cobb.  "Studies 
of  Inheritance  in  Rabbits."  Carnegie  Institution  Publications,  Wash- 
ington, No.  114,  1909. 


184 


GENETICS 


;t 


....b' 


/        2  34  56 


Offspring  ofZsnd  5 


<S    99666     9 


e  e   e    e  E  c 

e  e  e   e  6  6 

o  o  o  o  to  «> 

•»-  r-  t-  c~-  (B  lo 

CM             ' —  '- 


e 

e 

oo 


6669 dd 

E  E  E-  e  E  e 
e  e  s  e  E  E 
o   «X)   to  «o    p   w 

(S)    ^Q    iO    iO     t*»    r* 


Offsprin^of  land? 

Fig.  52.  —  A  case  of  three  generations  of  ear-length  in  rabbits,  a-b, 
average  ear-length  of  the  first  filial  generation  (Fi).  a'-b',  average 
ear-length  of  the  F2  generation  derived  from  1  and  7.  Data  from 
Castle,  in  collaboration  with  Walter,  Mullenix  and  Cobb. 

upon  the  ear-length  of  rabbits  which  included  70 
different  litters  of  rabbits  containing  341  individuals. 
In  none  of  these  experiments  could  the  blend  in  the 


BLENDING   INHERITANCE  185 

second  filial  generation  be  called  perfect,  but  it  may 
at  least  be  said  that  evidence  of  segregation,  that  is,  a 
return  to  one  or  the  other  of  the  parental  types,  was 
much  less  apparent  than  evidence  of  blending. 

Furthermore,  crosses  were  made  in  which  lop  ears 
of  various  fractional  lengths  were  obtained  as  desired, 
including  |,  |,  f ,  J,  f ,  J,  and  |  lengths.  Not  one  of 
these  fractional  lengths  apparently  segregated  in 
subsequent  generations  after  the  Mendelian  fashion, 
but  all  bred  approximately  true. 

Moreover,  ears  of  one  half  lop  length,  for  instance, 
were  obtained  in  three  ways  :  first,  by  crossing  full- 
length  lops  with  short-eared  rabbits  as  indicated  in 
the  first  cross  of  the  case  cited  above;  second,  by 
crossing  one  half  lop  lengths  together,  demonstrated 
by  the  second  cross  in  the  illustrative  case  given,  and 
third,  by  mating  J  and  f  lop  lengths.  Theoretically, 
I  and  I  as  well  as  f  and  f  lop  lengths  would  also  pro- 
duce I  lop  lengths,  for  in  all  of  the  crosses  that  were 
made  the  length  of  ear  behaved  in  a  blending  fashion. 

These  results  were  based,  not  upon  a  single  measure- 
ment of  each  specimen,  which  might  be  open  to 
considerable  error,  but  upon  daily  measurements 
from  the  time  the  rabbits  were  two  weeks  old  until 
their  ears  ceased  to  grow  at  about  twenty  weeks.  The 
growth  curves  drawn  from  these  daily  measurements 
showed  continually  an  intermediate  or  blending  condi- 
tion in  progeny  derived  from  diverse  parents. 

A  Mendelian  explanation  of  this  apparently  excep- 
tional case  of  blending  inheritance  has  been  suggested 
by   Lang   based   upon   the   result   of   Nilsson-Ehle's 


186  GENETICS 

discoveries  while  breeding  wheats  at  the  Agricultural 
Experiment  Station  of  Svalof  in  Sweden. 

8.  The  Nilsson-Ehle  Discovery 

Nilsson-Ehle  found  in  breeding  together  different 
strains  of  wheat  that  a  certain  wheat  with  brown 
chaff  crossed  with  a  white-chaffed  strain  yielded  only 
brown-chaffed  wheat  in  the  first  generation.  These 
heterozygous  or  hybrid  brown-chaffed  w^heats  when 
crossed  with  each  other  produced,  not  the  expected 
proportion  of  three  brown  to  one  white,  but  fifteen 
brown  to  one  white.  This  was  not  explainable  as  the 
chance  result  of  a  single  cross,  but  was  the  conclusion 
drawn  from  fifteen  different  crosses  all  of  the  same 
strains  that  yielded  a  total  progeny  of  1410  brown- 
chaffed  to  94  white-chaffed  plants,  which  happens 
to  be  exactly  the  proportion  of  fifteen  to  one. 

In  other  experiments  it  was  discovered  that  although 
dominant  red-kerneled  strains  of  wheat  crossed 
with  white-kerneled  varieties  usually  gave  the  three- 
to-one  proportion  upon  segregation  in  the  second 
filial  generation,  yet  one  particular  strain  of  red- 
kerneled  Swedish  wheat  in  the  second  generation 
gave  approximately  sixty-three  red  to  one  white- 
kerneled  strain. 

The  explanation  of  these  two  unexpected  results 
is  this.  In  the  case  of  brown-chaffed  wheat  there  are 
two  independent  determiners  for  the  character  of 
brown  color,  and  these  simply  follow  the  Mendelian 
laws  for  a  dihybrid,  w^hile  in  the  case  of  the  red- 
kerneled  wheat  there    are    three  independent  deter- 


BLENDING   INHERITANCE 


187 


miners  for  the  character  of  red  color,  each  of  which 
is  able  to  give  red  color  to  the  wheat.  Taken  together, 
these  three  determiners  behave  cumulatively,  follow- 
ing the  law  of  a  trihybrid. 

For  example,  if  a  brown-chaffed  wheat  with  the  for- 
mula BB\  in  which  B  and  B'  each  represent  a  brown- 
chaffed  factor,  is  crossed  w^ith  a  white-chaffed  wheat  of 
the  formula  bb\  in  which  b  and  b'  each  represent  the 
absence  of  B 
and  B^  respec- 
tively, then  all 
the  progeny  of 
this  cross  will  be 
brown-chaffed, 
having  the  zy- 
gotic formula 
BBW.  When 
upon  matura- 
tion the  gametes 
form  out  of  the 
germ-cells  from 
such  hybrids, 
the  following 
four  combina- 
tions are  pos- 
sible, and  no 
others :  BB\  Bb\  bB\  bb\  These  represent,  there- 
fore, the  possible  gametes  present  in  each  sex  of  the 
first  filial  generation,  and  upon  intercrossing  they  can 
combine  into  sixteen  possible  zygotes  to  form  the 
second  filial  generation,  as  shown  in  Figure  53. 


bb' 

Bb' 

bB' 

b  b 

bb' 

bb; 
bb 

@ 

Bb', 
BB 

(D 

bB' 
BB' 

© 

b  b' 
BB' 

© 

Bb 

BB' 
Bb' 

® 

B  b' 
Bb' 

® 

b  B' 
Bb' 

® 

b  b' 

Bb' 

® 

bB' 

BE' 

b  B' 

© 

B  b' 

b  b' 

© 

b  B' 
b  B' 

® 

b  h' 

b  B' 

® 

BB' 

bb'      b   b' 

® 

B  b' 
b  b' 

® 

b  B' 
b  b' 

® 

b  b' 
b  b' 

® 

Fig.  53.  —  Diagram  of  the  possible  combinations 
in  the  F2  generation  of  brown-chaffed  wheat 
according  to  experiments  of  Nilsson-Ehle.  B 
and  B'  are  cumulative  factors  for  the  brown- 
chaff  character,  b  and  b'  denote  the  absence  of 
B  and  B'  respectively. 


188 


GENETICS 


The  numbers  in  the  squares  indicate  how  many 
times  a  brown  determiner  is  present  in  each  zygote. 
It  will  be  seen  that  only  one  out  of  the  sixteen  possi- 
bilities lacks  a  brown-chaff  factor,  and  this  one  will 


n 


n 


n 


m 


u 


n 


n 


+ 


+ 


+ 


f      3      ^      /      0 

Number  of  doses  of  the  brown  determiner 

Fig.  54.  —  The  distribution  of  the  sixteen  possibilities  resulting  when  two 
similar  determiners  (brown-chaff)  act  together  as  a  dihybrid. 

consequently  produce  only  white  chaff,  while  the  re- 
maining fifteen  possibilities,  each  of  which  has  at  least 
a  single  determiner  for  brown,  will  all  yield  brown 
chaff. 

The   brown-chaff  factor,   moreover,   is    present   in 


BLENDING   INHERITANCE 


189 


varying  doses  among  these  fifteen  possil)ilities,  as  indi- 
cated by  the  numbers  in  the  squares.  It  is  evident, 
therefore,  that  several  shades  of  brown  will  be  rep- 


d< 

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6 

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5 

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000 
5 

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0 

Fig.  55.  —  Diagram  to  illustrate  Nilsson-Ehle's  case  of  trihybrid  red 
'  wheat.  The  large  screwheads  each  represent  a  single  determiner  for 
the  red  character.  The  small  screwheads  symbolize  the  absence  of 
the  red  character,  or  white.  The  number  in  each  square  indicates 
how  many  doses  of  the  "red"  determiner  is  present.  For  further 
explanation  see  text. 

resented  depending  upon  the  number  of  doses  of  the 
brown  determiner  in  each  instance. 

Figure  54  shows  how  these  different  shades  of 
brown  arrange  themselves  in  the  manner  of  a  fre- 
quency polygon  of  fluctuating  variation  with  the 
greatest  number  in  the  halfway  class  and  the  least 


190  GENETICS 

numbers  at  the  two  extremes.  In  this  instance 
six  out  of  sixteen  individuals  of  the  second  gen- 
eration theoretically  present  a  perfect  "blend"  be- 
tween the  original  brown-  and  white-chaffed  grand- 
parents, although  complete  segregation  has  actually 
occurred. 

The  same  explanation  holds  true  as  displayed  in 
Figure  55  for  the  trihybrid  case  of  red-  and 
white-kerneled  wheats  in  which  only  one  white- 
kerneled  to  sixty-three  red-kerneled  individuals  ap- 
pear in  the  second  filial  generation.  The  number 
of  red  determiners  in  each  zygote  is  indicated  by  the 
figure  at  the  bottom  of  each  square.  The  large  screw- 
head  symbols  with  vertical,  horizontal  and  diagonal 
slots  each  represent  an  independent  determiner  for 
red  kernel,  while  the  small  screw  heads  symbolize 
the  absence  of  each  of  these  determiners,  or  white 
kernel.  When  the  pure  strain  of  red-kerneled  wheat 
is  crossed  with  a  pure  strain  of  white-kerneled  wheat, 
the  first  generation  is  all   a  heterozygous  red  of  a 

Pure  red  -h  cuhite     =    Hybrid   red 

Fig.  56.  — The  result  of  crossing  white  wheat  with  trihybrid  red  wheat. 

somewhat  lighter  shade  than  the  original  pure  red 
strain. 

When  plants  of  this  heterozygous  sort  are  crossed 
together,  they  yield  plants  producing  red-kerneled 
and  white-kerneled  wheats  in  the  proportion  of  sixty- 
three  to  one.     The  sixty-three  kinds  of  red  wheats  are 


BLENDING   INHERITANCE 


191 


self-crossing  plants  of  the 
second  generation.  It  was 
to  be  expected  that,  if 
these  hybrid  wheats  of  the 
second  generation  carried 
one,  two,  three,  or  more 
determiners  for  a  red  kernel 
as  the  theoretical  tables  in 
Figures  55  and  57  demand, 
their  progeny  would  be 
distributed  with  reference 
to  the  number  of  red-  and 
white-kerneled  individuals, 
in  the  following  ratios  :  — 


m 


m 


m 


m 


m 


of  varying  degrees  of  redness  and  may  be  classified 
after  the  manner  of  fluctuating  variations  with  the 
greatest  number  of  kinds 
at  the  intermediate  degree 
between  pure  red  and  pure 
white.     (See  Figure  57.) 

In  order  to  test  whether 
the  sixty-four  kinds  of 
wheats  produced  in  the 
second  filial  generation,  as 
theoretically  displayed  in 
Figure  55,  really  contain 
separable,  though  indistin- 
guishable, determiners  for 
red-kernel,  Nilsson-Ehle 
produced  families  of  the 
third   filial   generation   by 


m 


« 


» 


a 


m 


m 


m 


m 


m 


m 


» 


m 


m 


# 


# 


# 


# 


# 


# 


# 


# 


# 


# 


# 


# 


# 


# 


# 


+ 


+ 


+ 


+ 


+ 


+ 


1 


0 


6  '    5 

r[G.  57.  —  Thedistributionof  the 
sixty-four  possibilitifs  in  tin-  F-i 
generation  when  three  similar 
determiners  act  together  as  a 
trihybrid. 


192  GENI^TICS 

3  red  to  1  white  when  1  determiner  for  red  is  present. 
15  red  to  1  white  when  2  determiners  for  red  are  present. 
63  red  to  1  white  when  3  determiners  for  red  are  present. 
All  red  to  no  white  when  4  or  more  determiners  for  red 
are  present. 

Among  seventy-eight  sample  families  of  the  third 
generation  inbred  to  test  this  theoretical  conclusion, 
the  actual  results  were :  — 

8  families  giving  the  ratio  of  3  red  to  1  white. 
15  families  giving  the  ratio  of  15  red  to  1  white. 

5  families  giving  the  ratio  of  63  red  to  1  white. 
50  families  giving  the  ratio  of  all  red  to  no  white.  . 

It  has  been  actually  demonstrated  therefore,  in 
the  case  of  this  particular  strain  of  wheat:  (1)  that  the 
factors  producing  red  kernel  are  several  in  number; 
(2)  that  they  act  independently  of  each  other  in 
heredity;  (3)  that  these  several  independent  factors 
segregate;  and  (4)  that  any  one  red  factor  acting 
alone  produces  a  "red"  result. 

The  Nilsson-Ehle  principle  of  cumulative  determin- 
ers has  been  confirmed  in  America  by  East  in  a  mas- 
terly series  of  breeding  experiments  upon  maize. 

In  connection  with  the  Nilsson-Ehle  principle,  it 
will  ,be  seen  that  the  possible  number  of  intergrades 
between  the  two  extremes  increases  rapidly  as  the 
number  of  duplicate  determiners  increases.  Thus 
with  six  duplicate  determiners  for  the  same  character 
present,  the  ratio  of  possible  dominants  to  recessives 
in  the  second  filial  generation  would  be  4095  to  1. 
The  reappearance  of  this  single  recessive  among  4095 


BLENDING   INHERITANCE  193 

dominants  would  be  extremely  unlikely,  and  it  might 
easily  be  mistaken  for  a  mutation  or  a  freak.  Appar- 
ent blends  of  all  intermediate  degrees,  however,  would 
be  sure  to  appear.  Yet  these  are  not  blends  in  the 
*' melting-pot"  sense  at  all,  but  strictly  cases  of  Men- 
delian  dominance  and  segregation. 

9.  The  Application  of  the  Nilsson-Ehle  Ex- 
planation TO  the  Case  of  Rabbit  Ear- 
length 

The  so-called  blending  rabbit  ears,  along  with 
other  similar  cases,  can  now  be  made  to  fall  into  line, 
as  pointed  out  by  Lang,  with  the  Mendelian  law  of 
segregation. 

If  we  assume  that  the  long  ear  of  the  lop  rabbit 
has  only  three  independent  but  equal  determiners  for 
excess  length,  the  case  becomes  one  of  Mendelian 
trihybridism  with  cumulative  factors,  which  works 
out  like  Nilsson-Ehle's  red-kerneled  wheat  in  the 
following  manner:  — 

In  general  the  average  for  full  lop  ear-length  may 
be  placed  at  220  mm.  and  for  the  ordinary  short- 
eared  rabbit  ^  at  100  mm.  The  difference,  or  the  ex- 
cess length  of  the  lop  ear,  is  120  mm.,  which,  according 
to  the  trihybrid  formula,  corresponds  to  the  six  doses 
of  the  character  symbolized  in  the  U])per  left-hand 
square  in  Figure  55  by  six  large  screw  heads,  three 

^  Not  the  Belgian  hare,  as  cited  in  the  illustration  given  in  Figure 
52.     The   Belgian    hare  has    typically   a    somewhat  longer    ear   than 
the  ordinary  short-eared  rabbit, 
o 


194 


GENETICS 


coming  from  each  parent  respectively.  If  all  of  these 
independent  determiners  are  equal  as  regards  excess 
ear-length,  each  factor  would  represent  an  excess  of 
20  mm.  above  the  normal  ear-length  found  in  short- 
eared  rabbits,  that  is,  — 


220  mm.  -  100  mm. 


6 


=  20  mm. 


When  according  to  this  computation  a  lop  (20 
mm.  X  6  +  100  mm.  =  220  mm.)  and  a  pure  short- 
eared  rabbit  (20  mm.  X  0  +  100  mm.  =  100  mm.) 
are  crossed,  if  imperfect  dominance  occurs,  which 
is  a  very  common  phenomenon,  it  is  true  that 
the  offspring  might  present  a  "blended"  appear- 
ance. If  now  these  cross-breds  of  the  first  gen- 
eration prove  to  be  trihybrids  with  respect  to  excess 
ear-length,  there  w^ould  be  sixty-four  possibilities 
in  their  progeny  segregating  out  just  as  in  the  red- 
kerneled  wheat. 

These  possibilities  would  be  arranged  in  the  fol- 
lowing f  reqviencies :  — 


Number  of  Excess  Ear- 

Number  of  Cases  occur- 

Total Length  in  Milli- 

length Determiners 

ring  OUT  OF  64 

meters  OF  Ears  resulting 

6 

1 

220 

5 

6 

200 

4 

15 

180 

3 

20 

160 

2 

15 

140 

1 

6 

120 

0 

1 

100 

BLENDING   INHERIJANCE  195 

Since  the  average  litter  among  rabbits  is  about 
five,  the  chances  that  these  five  rabbits  will  breed 
true  to  their  hybrid  parents  and  form  a  perfect 
blend  between  their  grandparents  is  20  out  of  64, 
while  the  chance  of  their  being  like  either  grand- 
parent is  only  one  out  of  64. 

It  should  be  noted  further  that  50  out  of  64,  or  77 
per  cent,  of  these  hybrids  of  the  second  filial  gen- 
eration would  have  an  ear-length  between  140  and 
180,  thus  approximating  a  "blend"  closely  enough 
to  be  so  classified  upon  a  casual  inspection. 

Moreover,  if  it  should  be  found  that  excessive 
ear-length  in  rabbits  is  due  to  more  than  three  dupli- 
cate determiners,  the  possibilities  of  getting  anything 
but  an  apparent  blend  would  be  much  decreased. 

The  fact,  furthermore,  that  the  fractional  ear- 
lengths  of  the  hybrid  rabbits  in  Castle's  experiments 
bred  approximately  true  in  the  second  and  subse- 
quent filial  generations,  may  also  be  explained  by 
the  Nilsson-Ehle  hypothesis. 

For  example,  half  lop  lengths,  according  to  this 
explanation,  are  those  with  three  doses  of  the  deter- 
miner for  excess  ear-length.  It  follows  that  the 
progeny  of  two  rabbits  each  carrying  three  doses  of 
a  determiner  will  likewise,  after  the  reduction  during 
the  maturation  of  the  germ-cells,  have  three  doses 

of  the  determiner  {—^ —  =  ^V 

It  would  be  interesting  to  breed  rabbits  having 
ears  of  one  eighth  lop  length  in  w^hich,  according  to 
the  foregoing  hypothesis,  there  would  presumably  be 


196  GENETICS 

present  only  a  single  determiner  for  excess  ear-length, 
with  ordinary  short-eared  rabbits  having  no  excess 
ear-length,  in  order  to  see  if  the  expected  Mendelian 
three-to-one  proportion  for  a  monohybrid  would  ap- 
pear in  the  progeny. 

10.   Human  Skin  Color 

Finally,  although  accurate  published  data  is 
wanting,  it  is  probably  true  that  skin  color  in  all 
kinds  of  hybrids  resulting  from  crosses  between 
negroes  and  whites  is  not  a  case  of  blending  inherit- 
ance, as  commonly  supposed,  but  rather  of  true 
Mendelian  segregation.  In  fact,  there  is  frequently 
visible  evidence  that  segregation  does  occur,  as  shown 
by  many  authentic  instances  where  the  offspring 
of  diversely  colored  parents  produce  children  with 
skin  color  of  different  shades. 

If  human  families  included  hundreds  of  offspring 
in  a  single  generation  instead  of  the  usual  number, 
the  problem  of  skin  color  in  man  could  doubtless 
be  quickly  solved  since  ratios  could  then  be  obtained 
large  enough  to  reveal  the  underlying  laws  of  inher- 
itance. 


CHAPTER  X 

THE    DETERMINATION    OF    SEX 
1.   Speculations,  Ancient  and  Modern 

From  the  earliest  times  the  desirabihty  of  con- 
trolhng  the  sex  of  an  unborn  child,  in  particular 
instances  at  least,  has  seemed  very  great.  Likewise 
the  wish  to  be  able  to  predetermine  sex  among  do- 
mesticated animals  has  made  breeders  quick  to  grasp 
at  every  clue  that  promised  success. 

There  has  been  no  want  of  speculations  concerning 
the  determination  of  sex.  J.  Arthur  Thomson, 
who  with  Professor  Geddes  wrote  "The  Evolution 
of  Sex"  in  1889,  says  :  "The  number  of  speculations 
as  to  the  nature  of  sex  has  been  well-nigh  doubled 
since  Dreylincourt,  in  the  eighteenth  century, 
brought  together  262  'groundless  hypotheses'  and 
since  Blumenbach  caustically  remarked  that  nothing 
was  more  certain  than  that  Dreylincourt's  own  theory 
formed  the  263rd.  Subsequent  investigators  have 
long  ago  added  Blumenbach's  theory  of  'Bildungs- 
trieb'  or  formative  impulse,  to  the  list."  It  maybe 
added  in  passing  that  the  hypothesis  of  the  deter- 
minative action  of  external  factors  upon  developing 
germ-cells  which  Geddes  and  Thomson  elaborated 
in  the  book  just  referred  to,  has,  in  its  turn,  accord- 
ing to  most  biologists,  joined  the  long  roll. 

197 


198  GENETICS 

Hippocrates  thought  that  sex  of  the  offspring 
depends  upon  the  relative  "vigor"  of  the  parents, 
while  Sadler  (1830)  concluded  that  the  relative  ages 
of  the  two  parents  is  the  determining  factor. 
Other  writers,  on  the  contrary,  have  thought  that 
the  age  of  the  mother  at  the  time  of  childbirth  deter- 
mines the  sex  of  the  offspring,  and  Thury  (1863),  in 
the  days  before  the  facts  of  maturation  were  known, 
ascribed  the  determinative  factor  to  the  relative 
degree  of  "ripeness"  of  the  egg  when  fertilized.  It 
was  once  assumed  also  that  the  right  ovary  or  the 
right  testicle  is  the  seat  of  one  sex  and  the  left  ovary 
or  left  testicle  of  the  other.  Galen,  who  did  the 
biological  thinking  for  several  centuries  of  mankind, 
asserted  that  the  right  side  of  the  body,  "being 
warmer"  than  the  left,  consequently  produces  males. 
Schenk  cites  a  most  amazing  bit  of  folk-lore  to  the 
effect  that:  "In  Servia  if  a  man  has  a  stye  on  his 
eyelid  he  comes  to  the  conclusion  that  his  aunt  is 
pregnant.  If  the  stye  is  on  the  upper  eyelid,  the 
child  will  be  a  male;    if  on  the  lower,  a  female." 

Modern  theories  of  sex  determination,  like  the 
earlier  speculations,  may  be  resolved  into  two  groups, 
namely,  those  which  depend  upon  controllable  ex- 
ternal or  environmental  factors  such  as  food,  climate, 
chemical  dosage  and  will  power,  and  those  which  de- 
pend upon  internal  factors  at  present  beyond  control. 

2.   The  Nutrition  Theory 

Of  external  factors  which  may  exert  a  moulding 
influence  upon  the  sex  of  the  offspring,  nutrition  is 


THE   DETERMINATION   OF   SEX  190 

possibly  the  most  potent.  This  factor  may  be 
conceived  to  act  either  upon  the  parent  previous 
to  the  maturation  of  the  germ-cells,  upon  the  germ- 
cells  themselves,  or  upon  some  susceptible  embryonic 
stage  of  the  life  cycle  subsequent  to  that  of  the  fer- 
tilized egg. 

It  has  been  suggested  that  since  the  egg  is  char- 
acterized by  possibly  a  more  advanced  metaboHc 
condition  than  the  sperm  due  to  the  presence  of  the 
nutritive  yolk,  consequently  the  more  yolk  or  nutri- 
tion there  is,  the  more  femaleness  will  characterize  the 
egg.  In  other  words,  femaleness  is  a  nutritive  condi- 
tion associated  in  the  egg  with  the  presence  of  yolk. 

A  generation  ago  Professor  Schenk  of  Vienna,  by 
controlling  the  nitrogenous  diet  of  certain  royal 
prospective  mothers,  gained  a  soothsayer's  reputa- 
tion as  a  prophet  of  sex  which  was  based  upon  several 
correct  predictions. 

Of  course,  any  prediction  of  sex  is  bound  to  turn 
out  correct  in  50  per  cent  of  the  cases,  regardless  of 
what  it  is  based  upon,  since  in  man  the  two  sexes 
are  approximately  equal  in  numbers.  Adherents 
of  all  sorts  of  theories,  therefore,  have  always  been 
able  to  produce  considerable  ''evidence"  to  sub- 
stantiate their  speculations,  however  crude  the  latter 
have  been. 

Statisticians  have  pointed  out  that  in  times  of 
unusual  hardship,  like  famine  or  war,  when  tlie 
amount  of  available  nutrition  for  pregnant  mothers 
is  presumably  reduced,  there  seems  to  be  a  prepon- 
derance of  males  born. 


200  GENETICS 

A  series  of  nutrition  experiments  upon  frogs  per- 
formed by  Born  ('81),  Pfluger  ('8^2),  and  Yung  ('85) 
showed  that  the  percentage  of  female  offspring,  which 
normally  is  slightly  over  fifty,  could  be  changed  to 
over  90  per  cent  by  regulating  the  food  supplied  to 
the  mother  before  the  egg-laying  period.  Cuenot 
and  King,  however,  working  independently,  repeated 
these  experiments  with  great  care,  taking  into  account 
all  the  eggs  that  were  laid  and  not  simply  the  ones 
that  developed,  and  both  obtained  negative  results. 
They  concluded,  therefore,  that  the  high  percentage 
of  female  tadpoles  appearing  in  the  initial  experi- 
ments was  due  to  a  greater  mortality  among  the  males 
and  not  to  the  transformation  of  possible  males  into 
females. 

There  seems  to  be  no  doubt  that  nutrition  may 
affect  the  percentage  of  those  which  reach  maturity. 
If  one  sex  requires  a  greater  amount  of  nutrition 
than  the  other  to  carry  out  successfully  the  more 
strenuous  metabolic  changes  in  its  life-cycle,  then 
unequal  percentages  between  the  sexes  of  the  sur- 
vivors resulting-  from  modified  nutrition  do  not  in 
any  way  help  to  solve  the  problem  of  determining 
the  sex  of  the  individual.  In  other  words,  the  elim- 
ination of  one  sex  through  modified  nutrition  does 
not  "determine"  the  other  sex. 

3.   The  Statistical  Study  of  Sex 

From  statistical  sources  it  has  been  ascertained 
that  ordinarily  there  is  produced  a  practical  equality 
in  the  numbers  of  the  two  sexes. 


THE   DETERMLXATIOX    OF   SEX  201 

Oesterlehen  in  Europe  summarized  the  daUi  for 
nearly  sixty  million  human  births  and  found  that 
an  average  of  106  males  are  born  to  every  100  fe- 
males. 

According  to  various  authorities,  the  relative  num- 
ber of  males  per  100  females  is  given  for  horses  as  99, 
for  cattle  94,  and  poultry  95,  while  in  pigs,  rabbits, 
pigeons,  and  greyhounds  the  corresponding  number 
of  males  is  slightly  over  100. 

This  practical  equality  of  the  sexes  in  all  sorts  of 
natural  environments  indicates  the  improbability  of 
the  assumption  that  external  conditions  determine 
sex. 

4.     MONOCHORIAL   TwiNS 

There  are  two  kinds  of  twins,  namely,  ordinary 
twins,  which  come  from  two  separately  fertilized 
eggs  each  inclosed  in  its  own  chorion,  and  "identical 
twins,"  that  have  their  origin  in  one  egg  which  is  in- 
closed in  one  chorion.  Of  the  former,  something  like 
30  per  cent  in  man  are  reported  as  being  of  two  sexes, 
thus  showing  that  it  is  neither  nutrition  nor  envi- 
ronment which  determines  sex.  Usually  when  twins 
are  of  the  same  sex,  they  exhibit  as  great  a  range  of 
difference  in  mental  and  physical  traits  as  do  ordinary 
children  of  the  same  fraternity  born  at  different 
times,  but  occasionally  "identical  twins"  are  born, 
and  such  monochorial  twins  are  always  of  ihe  same 
sex.  This  is  evidence  that  sex,  like  other  somatic 
characters,  is  determined  in  the  germplasm  at  the 
time  of  fertilization. 


202  GENETICS 

Similarly,  in  the  clialcid  fly,  Ageniapsis,  sl  chain  of 
embryos  is  formed  from  a  single  egg,  and  these,  ac- 
cording to  Marschal,  are  all  of  the  same  sex. 

Newman  and  Patterson  also  have  shown  that  in 
the  armadillo,  Tatusia,  there  are  customarily  produced 
four  young  within  a  single  chorion,  all  of  which  are 
of  the  same  sex. 

These  facts  point  toward  the  conclusion  that  the 
determination  of  sex  takes  place  at  the  time  of  fer- 
tilization. 

5.   Selective  Fertilization 

Within  the  last  ten  years  considerable  evidence 
has  been  collected  in  support  of  the  supposition  that 
sex  is  a  Mendelian  character.  Mendel  himself, 
without  elaborating  this  idea  into  a  definite  hypothe- 
sis, suggested  the  probability  that  sex  is  a  heritable 
character  behaving  in  the  same  way  as  other  herit- 
able characters. 

In  1903  Castle  published  a  paper  ^  in  which  a 
tentative  explanation,  since  abandoned,  of  the  phe- 
nomenon of  sex  determination  was  advanced,  based 
upon  three  assumptions  :  first,  that  all  germ-cells  are 
heterozygous  for  sex  and,  therefore,  upon  maturation 
there  are  formed  both  male  and  female  eggs  as  well 
as  male  and  female  sperms ;  second,  that  in  fertili- 
zation the  gametes  always  unite  with  their  opposites 
so  far  as  sex  is  concerned  ajid  never  with  their  like, 
with  the  result  that  each  fertilized  egg  must  carry 

1  Castle,  W.  E.,  "The  Heredity  of  Sex."  Bull.  Mus.  Comp.  Zool., 
Harvard,  Vol.  XL,  No.  4,  1903. 


THE   DETERMINATION   OF  SEX 


203 


determiners  for  both  sexes  and  be  lieterozygous,  as 
indicated  in  Figure  58;  and  third,  tliat  the  character 
of  sex  follows  the  law  of  alternative  dominance,  ac- 
cording to  which  in  the  male  offspring  the  nude 
determiner  dominates  M(F),  while  in  the  female 
the  female  dominates  {M)F. 

This  hypothesis  is  simply  an  attempt  to  explain 
the  numerical  equality  of  the  sexes,  and  also  the  fact 
that  the  determiner  for  the  opposite  sex  may  be  car- 


MALC 


Gametes 


FEMALE. 


M(r)    M(M)    (DF     r(M) 

MALE     -  DO  NOT   OCCUR  -    FEMALE 


ZYGOTES 

Fig.  58.  — Diagram  to  show  Castle's  1903  theory  of  the  heredity  of  sex. 


ried  by  either  parent,  but  it  leaves  unanswered  the 
question  of  what  causes  ''selective  fertilization" 
and  "alternative  dominance." 

There  appears  to  be  some  evidence  that  selective 
fertilization,  which  was  assumed  in  Castle's  1903 
theory,  may  actually  occur  under  certain  circum- 
stances. For  example,  homozygous  or  pure  yellow 
mice,  that  is,  mice  with  a  duplex  determiner  for 
yellow  color,  are  not  known.  In  breeding,  all  kinds 
of  yellow  mice  behave  as  if  heterozygous  or  simplex 
with  respect  to  yellow  color,  for  when  any  two  yellow 
mice  are  bred  together,  they  produce  a  certain  per- 
centage of  recessives  which  would  not  happen  if  they 


204  GENETICS 

were  pure  yellow.  In  a  Mendelian  monohybrid  cross, 
as  has  been  previously  pointed  out,  the  expectation  is 
that  in  the  second  generation  one  fourth  of  the  offspring 
will  be  recessives  {DR  X  DR  =  DD -\-2DR-\-  RR) ,  but 
when  yellow  mice  are  bred  together,  the  percentage  of 
recessives  approximates  one  third  instead  of  one  fourth. 
This  apparent  exception  to  the  Mendelian  ratio  finds 
an  explanation,  however,  when  it  is  assumed  that  selec- 
tive fertilization  takes  place  in  such  a  cross,  and  thus, 
since  a  D  gamete  never  unites  with  another  D  gamete, 
but  always  with  its  opposite,  R,  pure  yellow  mice  are 
unknown. 

This  supposition  is  further  supported  by  the  fact 
that  the  litters  of  young  from  yellow  mice  are,  on  an 
average,  only  three  fourths  as  large  as  normal  litters 
of  mice,  which  is  exactly  what  would  be  expected 
if  one  fourth  of  the  possible  gametic  combinations 
{DD)  fail  to  produce  offspring. 

Castle's  tentative  explanation  of  the  determina- 
tion of  sex  at  least  breaks  away  from  the  old  concep- 
tion that  the  sperm-cell  produces  male  offspring  and 
the  egg-cell,  females.  It  agrees,  too,  with  Darwin's 
idea  that  both  sexes  are  present  in  each  individual 
with  one  sex  latent.  In  certain  parthenogenetic 
rotifers,  aphids  and  daphnids,  both  sexes  are  plainly 
present  in  the  female,  since  two  kinds  of  easily  dis- 
tinguishable eggs  are  produced,  one  of  which  develops 
into  males  and  the  other  into  females  without  fer- 
tilization or  any  kind  of  a  union  with  a  sperm-cell. 


THE   DETERMINATION   OF  SEX 


205 


6.   The  Neo-Mendelian  Theory  of  Sex 

Correns  (1906)  avoids  the  difficulties  of  alternative 
dominance,  which  Castle's  hypothesis  offers,  by  sup- 
posing that  one  par- 


Type 


I 


ZYGOTES 


(Female)  QcT 
(Male)     Cfcf 


n 


(Female! 
(Male) 


99 
9cf 


GAMETES 


9 


(F. 


9 


:)  ( 


-h 


Male) 


W 

(Female) 


QcT 

(Male) 


Fig.  59.  —  Diagram  to  show  tho  neo-Men- 
delian  theory  of  the  heredity  of  sex,  using 
sex  symbols. 


ent  only  is  hetero- 
zygous with  respect 
to  sex,  and  this  sup- 
position is  becom- 
ing more  and  more 
probable  as  evidence 
accumulates.  Ac- 
cording to  this  idea, 
there  are  two  types 
of  cases,  one  when  the  female  is  the  heterozygous 
parent  and  the  other  when  the  male  is  the  hetero- 
zygous parent,  as  represented  in  Figure  59. 

The  formulse  for  these  types  may  be  expressed  in 
the  nomenclature  of  the  presence  and  absence  theory, 

as  follows  (Fig.  60), 
in  which  the  sym- 
bol X  represents  the 
female  determiner 
in  the  heterozygous 
case  of  type  I,  and 
XX  the  female  de- 
terminer when  the 
male  is  the  hetero- 
zygous parent. 
The  formulae  may  be  still  further  modified,  accord- 
ing to  Morgan,  for  the  satisfaction  of  those  who  ob- 


Type 

Zygotes 

Gametes 

(Female) 

X 

o 

X 

-H      O 

I 

(Male) 

o 

o 

o 
xo 

(Female 

H-      O 

oo 

)      ( Male ) 

n 

(Female) 

X 

X 

x 

+      X 

(Male) 

X 

o 

X 

XX 

( Female 

+      O 

XO 

)      (Male/ 

Fig.  60.  —  Diagram  to  show  the  neo-Men- 
delian  theory  of  the  heredity  of  sex  accord- 
ing to  the  presence  and  absence  hypothesis. 


206 


GENETICS 


ject  to  regarding  the  male  factor  as  nothing  positive, 
but  simply  the  absence  of  femaleness,  by  assuming 
that  a  universal  factor  of  maleness  (m)  is  present  in 
all  cases,  as  shown  in  Figure  61. 

Thus  in  type  I  of  this  scheme  it  is  only  when  the 
dominant  female  factor  F  is  entirely  absent  that  male- 
ness becomes  expressed  in  the  somatoplasm,  while  in 

type  II  it  is  neces- 
sary to  have  a  double 
dose  of  the  factor  F 
in  order  to  produce 
a  female,  since  a 
single  dose  results  in 
a  male. 

All  of  these  three 
theoretical  schemes 
agree  in  assuming 
that  one  sex  is  hetero- 
zygous, while  the  other 
is  homozygous  and  that  femaleness  is  the  result  of 
an  added  factor  in  excess  of  maleness. 

The  evidence  for  these  conclusions  has  been  ob- 
tained chiefly  from  four  sources  :  first,  from  a  mi- 
croscopical examination  of  the  germ-cells ;  second, 
from  castration  and  regeneration  experiments ;  third, 
from  the  results  of  hybridization  in  "sex-limited 
inheritance";  and  fourth,  from  the  behavior  of 
hermaphrodites   in   heredity. 


Type 

Zygotes 

GrAMETES 

I 

(Female)    Fmfm 

Fm    -1-    fm 

(Male)      fmfm 

fm     -\-     fm 

Fmfm           fmfm 
(Fema\e)       (Ma\e) 

n 

(Female)    FrnFm 

Fm    -\-     Fm 

(Mole)      Fmfm 

Fm    -f     fn\ 

FrnFm           Fmfm 
(Fema\e)       (Male) 

Fig.  61.  —  Diagram  to  show  the  neo-Men- 
delian  theory  of  the  heredity  of  sex  with 
Morgan's  modification,  making  male- 
ness (m)  present  as  a  positive  character 
in  every  gamete. 


THE   DETERMINATION   OF  SEX  207 

a.    Microscopical  Evidence 
A,  1.    The  "X"  Chromosome 

In  1891  Henking  called  attention  to  the  presence 
of  two  kinds  of  spermatozoa  in  the  firefly,  Pyrrho- 
corisy  and  later  McClung  (1901),  in  studying  the 
spermatogenesis  of  the  grasshopper,  discovered  a 
similar  phenomenon  with  respect  to  the  chromosomes 
of  its  spermatozoa.     Soon  after,  Stevens  and  Wilson 


Fig.  62.  —  Diagram  to  show  how  numerical  equality  of  the  sexes  results 
when  one  parent  is  homozygous  (the  female  in  this  instance)  and  the 
other  is  heterozygous  for  the  sex  character. 

working  independently  on  various  species  of  insects, 
and  Boveri,  on  sea-urchins,  found  that  when  the 
male  is  characterized  by  two  kinds  of  sperm-cells, 
one  of  which  has  an  "extra"  chromosome  (the  so- 
called  "accessory"  or  "a:"  chromosome),  while  the 
other  does  not,  the  female  of  the  same  species,  upon 
maturation  of  the  eggs,  produces  mature  eggs,  all  of 
which  possess  one  "a:"  chromosome.  The  result 
of  this  heterozygous  condition  of  the  male  and  homo- 
zygous condition  of  the  female  with  respect  to  the 
X  chromosome  is  the  theoretical  equality  of  the 
sexes  among  the  individuals  formed  by  their  union,  as 


208  .    GENETICS 

shown  in  Figure  62  or  in  type  II  of  Figures  59,  60 
and  61. 

It  will  be  seen  that  when  a  male  gamete  bearing 
an  X  chromosome  unites  with  a  female  gamete  also 
bearing  an  x  chromosome,  the  outcome  is  a  fertilized 
egg  containing  xx  chromosomes.  Such  an  egg  is  con- 
sequently homozygous  for  sex,  and  will  develop  into 
a  female  individual.  In  the  same  way  when  a  male 
gamete  lacking  an  x  chromosome,  as  half  the  gametes 
derived  from  a  heterozygote  do,  unites  with  a  female 
gamete  bearing  an  x  chromosome,  as  all  gametes 
from  a  homozygote  must  do,  then  the  fertilized  egg 
will  be  heterozygous,  carrying  only  one  x  chromosome, 
and  will  develop  into  a  male  indi\adual. 

The  chromatin  difference  between  the  two  sexes 
may  be  qualitative,  as  Wilson  holds,  or  quantitative, 
as  Morgan  assumes,  but  in  either  case  it  seems  cer- 
tain that,  with  difference  in  sex,  there  is  invariably 
associated  a  definite  difference  in  the  character  of 
the  chromosomes  present  in  the  germ-cells. 

These  conclusions  have  been  abundantly  con- 
firmed in  various  species  by  a  large  number  of  inde- 
pendent w^orkers,  and  are  now  well  established  as 
a  part  of  biological  science.  In  fact,  it  is  not  at 
all  unusual  to  find  the  technical  confirmation  of  the 
X  chromosome  theory  given  as  a  part  of  the  routine 
class  work  in  university  courses. 

A,  2.    Various  Forms  of  X  Chromosomes 

The  extra  chromosome  in  different  species  may 
assume  various  forms  or  degrees  of  complexity.     It 


THE   DETERMINATION   OF  SEX  209 

may  be  either  single  or  multiple.  It  may  be  paired 
before  maturation  with  its  absence,  or  with  an  unlike 
("i/")  chromosome.  It  may  be  linked  inseparably 
with  some  one  of  the  ordinary  chromosomes  (auto- 
somes), or  resemble  the  autosomes  so  closely  that  its 
presence  can  only  be  assumed  from  analogy  with 
other  cases,  and  not  definitely  determined  at  all. 

In  all  of  these  cases,  however,  there  is  one  point 
of  likeness,  and  that  is  that  there  always  seems  to 
be  additional  chromatin  material  associated  with  the 
female  sex. 

The  reason  for  this  may  lie  in  the  more  highly 
metabolic  requirements  of  the  female,  who  must 
produce  yolk  or  provide  in  some  way  for  the  main- 
tenance of  the  young  in  addition  to  furnishing  half 
of  the  germinal  heritage. 

In  the  microscopical  evidence  on  this  point  there 
is  one  apparent  exception  to  the  rule  that  females 
are  homozygous  and  males  heterozygous  with  respect 
to  sex.  Baltzer  (1910)  found  that  in  one  of  the  sea- 
urchins  an  extra  sex  chromosome  is  associated  with 
the  female  sex,  so  that  two  kinds  of  mature  eggs  are 
produced  upon  maturation  and  only  one  kind  of 
sperm-cells.  In  other  words,  in  this  case  the  female 
is  heterozygous  for  sex  and  the  male  homozygous, 
instead  of  the  reverse  which  is  true  for  all  other 
forms  thus  far  microscopically  investigated. 

Such  cases  as  this  of  the  sea-urchin  are  theoreti- 
cally provided  for  in  the  formulae  under  type  I  given 
above  in  Figures  59,  60  and  61. 


210  GENETICS 

Ay  3.  Sex  Chromosomes  in  Parthenogenesis 

The  behavior  of  the  chromosomes  in  cases  of 
parthenogenesis,  where  the  union  of  an  egg-cell  and 
a  sperm-cell  are  not  necessary  for  the  production 
of  a  new  individual,  throws  additional  light  upon 
the  relation  between  chromosomes  and  sex  determi- 
nation. 

For  instance,  among  the  social  Hymenoptera,  bees, 
ants,  wasps,  etc.,  the  "queen"  produces  eggs  which 
upon  maturation,  if  unfertilized,  develop  into  males 
or  drones,  all  of  whose  cells  contain  a  reduced  amount 
of  chromatin  (Fig.  63).  It  is  only  when  sexual  repro- 
duction occurs  through  the  union  of  a  mature  egg- 
cell  and  a  mature  sperm-cell  or  spermatozoan,  that 
the  full  complement  of  chromatin  is  restored  to  the 
fertilized  egg  and  females  are  again  produced. 

Castle  says  :  "In  all  known  cases  of  parthenogenesis 
the  female  is  in  the  duplex  (2  n)  condition,  and  the 
male  is  in  the  simplex  (n),  or  partially  duplex  (2  n  —  1 
condition.  The  female  in  all  cases  has  the  greater 
chromatin  content." 

b.  Castration  and  Regeneration  Experiments 

Certain  characters  which  are  known  as  "secondary 
sexual  characters,"  such  as  the  ornamental  plumage 
in  male  birds,  the  beard  in  man  or  the  sting  in  worker 
bees,  are  often  associated  with  a  definite  sex.  When  an 
individual  is  castrated,  it  is  quite  common  not  only 
for  these  peculiar  secondary  sexual  characters  to  dis- 
appear, but  also  for  the  secondary  sexual  characters  of 


THE   DETERMINATION   OF  SEX 


211 


9  SOMA  (Queen) 


LAR    BODY 


OCYTC 


—   Polar  body 

^—    PcLAR    BODY 

Mature  cgo 


5perm  cells 


©fERTlUZEO 
EGG 


Fig.  63.  —  Diagram  of  the  heredity  of  sex  in  bees,  ants  and  wasps.  The 
outline  chromosomes  represent  sample  somatic  chromosomes.  The 
solid  black  chromosomes  stand  for  sex.  The  female  has  two  sex 
chromosomes  while  the  male  has  but  one. 


the  opposite  sex  to  develop  to  a  certain  degree  in 
their  stead.  This  indicates  that  the  determiners  for 
sex  are  intimately  associated  with  those  for  the  sec- 


212 


GENETICS 


ondary  sexual  characters,  and  also  that  the  determin- 
ers for  the  opposite  sex  are  often  present  in  a  latent 
condition,  or,  in  other  words,  that  the  organism, 
either  male  or  female,  is  heterozygous  with  respect  to 
sex. 

If  a  female  of  the  annelid  worm  Ophiotrocha,  for 
example,  is  cut  in  half,  it  is  effectually  castrated,  be- 


Male 


PardSiliwllij  castrated  male 


Female 


Fig.  64.  —  The  crab,  Inachus,  parasitized  by  the  cirripede,  Sacculina. 
Evidence  that  a  Mendelian  sex  determiner  is  correlated  with  "sec- 
ondary sexual  characters  "  and  that  the  male  is  heterozygous  for  sex 
while  the  female  is  homozygous.    After  Smith. 

cause  the  ovaries  are  in  the  posterior  part  of  the  body. 
It  has  the  power  of  regeneration,  however,  but  when 
a  new  posterior  part  is  formed,  it  contains,  not  female, 
but  male  reproductive  organs.  The  worm  is,  there- 
fore, now  a  male,  as  shown  by  the  presence  of  testes 
instead  of  ovaries,  proving  that  it  was  originally 
heterozygous  with  respect  to  sex,  carrying  one  sex 
latent. 


THE   DETERMINATION   OF  SEX  213 

According  to  Smith,  parasitic  castration  is  performed 
on  the  crab  Inachus,  which  is  found  in  the  Bay  of 
Naples,  by  a  cirripede,  Sacculina.  The  male  crab 
of  this  species  has  one  large  claw  and  a  narrow  abdo- 
men, while  the  female  has  no  large  claw,  but  a  broad 
abdomen.  When  Sacculina  parasitizes  the  female,  the 
secondary  sexual  characters  of  the  female  are  stunted, 
but  not  materially  changed.  When,  on  the  contrary, 
the  male  is  parasitized,  it  not  only  loses  its  distinctive 
large  claw  in  subsequent  molts,  but  it  also  takes 
on  the  broad  abdomen  of  the  female  (Fig.  64).  This 
apparent  anomaly  is  quite  explainable  upon  the  as- 
sumption that  the  female  is  homozygous  for  sex 
and  the  accompanying  secondary  sexual  characters, 
while  the  male  is  heterozygous.  When  maleness  is 
destroyed  in  the  male  by  the  castrating  parasite, 
therefore,  the  femaleness  that  is  latent  in  this  sex 
becomes  manifest  through  the  appearance  of  female 
secondary  sexual  characters ;  but  when  the  female  is 
castrated,  no  other  secondary  sexual  characters  than 
those  already  present  make  their  appearance,  since 
only  femaleness  is  present  in  the  homozygous  female 
sex. 

c.  Sex-limited  Inheritance 

Additional  evidence  that  sex  is  a  character  depend- 
ing upon  determiners  which  behave  in  Mendelian 
fashion  is  furnished  by  what  is  called  sex-limited 
inheritance.  There  are  certain  characters  known  as 
sex-limited  characters  that  are  in  no  sense  to  be  con- 
fused with  secondary  sexual  characters  which  appear 
to  be  always  linked  with  the  determiner  for  either  one 


214 


GENETICS 


sex  or  the  other.     They  are,  therefore,  well  described 
by  the  term  *' sex-limited." 


(1)   Color-blindness 

This  phenomenon  may  be  illustrated  by  the  in- 
heritance of  human  color-blindness,  a  character  which 
appears  to  be  linked  with  the  determiner  for  sex.  It 
requires  a  duplex,  or  homozygous,  dose  of  the  deter- 
miner for  color-blindness  to  produce  a  color-blind 
female,  while  only  a  simplex,  or  heterozygous,  dose  is 


Gametes    (x 


f:  90  9@ 


Fig.  65.  —  General  diagram  for  sex-limited  inheritance.  The  underscored 
symbol  (21)  represents  a  sex  determiner  with  some  other  character 
(as  color-blindness)  linked  with  it. 

needed  to  produce  a  color-blind  male.  These  facts 
agree  perfectly  with  the  idea  that  the  female  is  homo- 
zygous and  the  male  heterozygous  with  respect  to 


THE   DETERMINATION   OF   SEX  215 

sex,  and  that  the  factor  for  color-blindness  is  Hnked 
with  the  determiner  for  sex.  Sex-Hmited  inheritance, 
as  shown  in  this  case,  may  be  ilhistrated  l>y  the  (ha- 
gram  on  the  opposite  page  (Fig.  65)  in  which,  for  the 
sake  of  simphcity,  only  sex  chromosomes  and  the  de- 
terminers for  color-blindness  are  represented.  Under- 
scored 2<  represents  a  color-blind  determiner  linked 
to  a  sex  chromosome. 

From  this  diagram,  which  agrees  substantially 
with  the  facts,  it  is  apparent  that  a  color-blind  male 
mated  to  a  normal  female  will  produce  no  color-blind 
offspring,  although  the  females  will  be  "carriers" 
of  color-blindness,  that  is,  will  possess  the  factor  in 
simplex  form  and  will,  therefore,  carry  it  for  the  fe  in  ale 
in  a  latent  condition. 

The  sons  of  such  a  mating  having  a  normal  mother 
and  a  color-blind  father  will  be  absolutely  free  from 
the  defect  and  cannot  produce  color-blindness  in  any 
of  their  offspring  when  mated  with  a  normal  strain. 
If,  however,  the  "carrier"  daughters  from  such  a 
parentage,  who  are  genotypically  heterozygous  for 
color-blindness  but  phenotypically  normal,  mate  with 
normal  individuals,  the  expectation  is  that  one  half 
of  the  sons,  and  none  of  the  daughters  will  be  color- 
blind, but  that  one  half  of  these  daughters  will  carry 
the  color-blind  determiner  in  simplex  form,  that  is,  in 
a  condition  ineffective  for  producing  color-blindness 
in  female  individuals. 

All  of  the  various  possibilities  in  the  inheritance  of 
color-blindness  according  to  the  sex-limited  interpre- 
tation are  indicated  in  the  following  table :  — 


216 


GENETICS 


Parents 

Expected  Offspring 

Normal 

9 

Color-blind 

Color-blind 

9 

Carrier 

Normal 

Carrier 

^  color-blind 
1  normal 

^  carrier 
^  normal 

Color-blind 

Normal 

Normal 

Carrier 

Color-blind 

Color-blind 

Color-blind 

Color-blind 

Color-blind 

Carrier 

^  color-blind 
^  normal 

^  color-blind 
^  carrier 

(2)   r/^^  English  Currant-worm 

A  famous  case  of  sex-limited  inheritance  is  that  of 
the  EngKsh  currant- worm,  Abraxas,  which  occurs  in 
two  varieties,  viz..  Abraxas  grossulariata  and  Abraxas 


Fig.  66.  —  Abraxas  grossulariata,  the  English  currant-moth,  and  (on  the 
right)  its  paler  lacticolor  variety.     From  Punnett's  "  Mendelism." 

lacticolor  (Fig.  66).  The  Hghter-colored  lacticolor 
is  recessive  to  the  darker-colored  grossulariata  variety 
and  has  been  found  in  nature  associated  only  with  the 
female  sex. 


THE   DETERMINATION   OF  SEX 


217 


Doncaster  and  Raynor,  in  1908,  published  the  results 
of  various  crosses  between  these  two  varieties  which 
demonstrate  clearly  that  sex  is  a  Mendelian  character 
and  that,  in  this  instance,  maleness  is  homozygous 
and   femaleness    heterozygous    with    the    determiner 

Key  to  Symbols 


Phenotype 


Gross.c? 


Gross,  c? 


Lact.c? 


GR0SS.9 


Lact.^ 


Conslilulion  with 
respect  to  ihe 

GROSSULARIATA 


factor 


Dupl  ex 


Simplex 


fSlulliplex 


Simplex 


Nulliplex 


Genotype 


(joi  Ooi 


Iqi  Ooi 


()0I  Jfll 


Oof  flo 


Ooi  Do 


Gametes 


01 


Fig.  67.  —  Key  to  the  symbols  employed  in  Figures  68-71.  The  outline 
symbols  represent  samples  of  the  autosomes  or  somatic  chromosomes. 
The  black  symbols  stand  for  the  "extra"  or  sex  chromosomes.  G 
above  a  black  symbol  indicates  the  grossulariata  factor  linked  with  a 
sex  chromosome.  The  variety  lacticolor  occurs  whenever  the  grossu- 
lariata factor  is  absent. 

for  maleness  linked  with  the  factor  producing  the 
variety  grossulariata.  A  study  of  Figures  67-71 
will  make  this  case  clear.  Outline  symbols  represent 
ordinary  chromosomes  or  autosomes,  several  of  which 
are  omitted  for  sake  of  clearness.  The  black  sym- 
bols represent  sex  chromosomes.     The  letter  G  placed 


218  GENETICS 

above  a  black  symbol  represents  the  grossidariata 
factor  linked  with  a  sex  chromosome.  The  variety 
lacticolor  occurs  whenever  the  factor  for  grossulariata 
is  absent.  In  this  case  two  sex  determiners  are  neces- 
sary to  produce  a  male,  and  only  one  to  produce  a 
female.  In  the  following  theoretical  diagrams  the 
actual  number  of  offspring  obtained  by  Doncaster 
and  Raynor  in  each  cross  is  indicated  outside  the  circles 
that  represent  the  zygotes,  and  the  parenthetical 
numbers  refer  to  the  five  kinds  of  individuals  cata- 
logued in  Figure  67. 

In  the  first  cross  (Fig.  68)  where  a  lacticolor  female 
(5)  and  a  grossulariata  male  (1)  were  bred  together, 
the  entire  progeny  was  grossidariata  in  character  with 
an  approximate  equality  between  the  sexes,  that  is, 
45  males  (2)  to  50  females  (4). 

When  these  hybrid  grossulariata  individuals,  (2)  and 
(4),  were  mated  with  each  other  in  Cross  2  (Fig.  69), 
the  character  of  grossulariata  appeared  again  in  both 
sexes,  (1),  (2),  and  (4),  while  the  character  lacticolor 
was  confined  as  usual  to  females  alone  (5).  It  was 
only  when  grossulariata  hybrid  males  (2)  were  crossed 
back  to  lacticolor  recessive  females  (5)  in  Cross 
3  (Fig.  70)  that  individuals  of  both  varieties  and 
both  sexes  appeared,  (2),  (3),  (5),  (4),  in  practically 
the  expected  equal  numbers,  namely,  63,  65,  70,  62. 
The  lacticolor  male  (3)  obtained  by  bringing  together 
the  two  sex  determiners  necessary  for  maleness,  each 
of  which  had  been  dissociated  through  the  foregoing 
crosses  from  the  sex-limited  grossulariata  factor,  was  en- 
tirely new  to  science,  never  having  been  found  in  nature. 


THE   DETERMINATION   OF  SEX 


219 


Cro  ss  1 


GAMETE5 


Zygotes 


1-5 

Gross,  d* 


(2) 


(0 
Gross  cf 


Gross.  ^ 
(4) 


Fig.  68. — The  formation  of  heterozygous  grossulariata  individuals,  both 
male  and  female,  by  crossing  pure  grossulariata  males  with  lacticolor 
females. 


Cro  5s  £ 


(2) 

Gross. cf 


GRoss.d     GRoss-d*  Lact.  9     Gross,  (f 

(/)  (Z)  (5)  (4) 

Fig.  69.  —  The  cross-breeding  of  heterozygous  grossulariata  individuals. 


220  GENETICS 

Finally,  when  these  newly  made  lacticolor  males  (3) 
were  crossed  with  heterozygous  grossulariata  females 
(4)  (Fig.  71),  the  proportion  of  sexes  was  approxi- 
mately equal,  as  expected,  that  is,  145  males  to  130 
females,  but  all  of  the  males  were  of  the  heterozygous 
grossulariata  type  (2)  and  all  of  the  females  of  the  re- 
cessive lacticolor  type  (5),  showing  a  return  to  the  sex- 
limited  condition.  All  of  these  curious  results  find  a 
satisfactory  and  complete  explanation  in  the  assump- 
tion, first,  that  sex  is  a  Mendelian  character  carrying 
tw^o  determiners  for  maleness  and  one  for  femaleness  ; 
and,  second,  that  the  determiner  for  the  character  of 
grossulariata  when  present  is  always  linked  to  the  sex 
determiner. 

This  case  is  of  particular  interest,  since  it  agrees 
with  the  microscopical  evidence  already  referred  to 
in  connection  with  the  chromosomes  of  Baltzer's  sea- 
urchins,  in  which  the  male  was  likewise  homozygous 
and  the  female  heterozygous  with  respect  to  sex. 

The  chromosomes  of  Abraxas  present  certain  techni- 
cal difficulties  which  at  present  have  not  been  over- 
come, so  that  we  do  not  yet  know  whether  the  evi- 
dence of  the  heterozygous  character  of  one  sex  and  the 
homozygous  character  of  the  other,  obtained  from  the 
breeding  experiments  of  Doncaster  and  Raynor,  will 
be  confirmed  upon  a  microscopic  examination  of  the 
chromosomes  in  the  germ-cells. 

(3)    The  Behavior  of  Hermaphrodites  in  Heredity 

Certain  plants  occur  in  monoecious  form,  that  is,  as 
hermaphrodites,  and  also  in  dioecious  form,  that  is,  with 


THE   DETERMINATION   OF   SEX 


221 


Cross  3 


(2) 
Gross  d 


Gross,  d*    Lact.  c? 
(2)  (3) 


(Ooioi) 

7(X ^         6Z-' 

Lact.  5     Gross  ^ 

(5)  m 

Fig.  70.  —  Heterozygous  grossulariata  male  crossed  with  lacticolor  female. 
One  fourth  of  the  progeny  are  lacticolor  male,  not  known  to  occur  in 
nature. 


Cross  4 


GROSS   d"  tACT.    ^ 

(2.^  (5) 

Fig.  71.  —  Back  cross  of  lacticolor  male  with  grossulariata  female  produc- 
ing the  original  sex-limited  condition  in  which  all  the  females  are  of 
the  lacticolor  type.  Data  for  Figures  68-71  from  Doncaster  and 
Raynor. 


£22  GENETICS 

the  sexes  on  separate  plants.  Among  such  dimorphic 
plants,  Bryonia  in  particular  has  been  investigated  by 
Correns  and  Lychnis  by  Shull.  Without  describing 
the  crosses  made  in  their  experiments  in  detail,  it 
may  be  stated  that  when  dioecious  types  are  recipro- 
cally crossed  with  hermaphroditic  forms,  the  result- 
ing progeny  indicate  plainly  that  one  sex  is  homozy- 
gous while  the  other  is  heterozygous  with  respect  to 
the  sex  character.  This  confirmatory  evidence  is 
quite  in  line  with  that  already  brought  forward  that 
sex  is  a  Mendelian  character  the  determiners  of  which 
are  carried  in  the  germplasm. 

7.   Conclusion 

The  evidence  thus  far  obtainable  from  all  sources 
points  to  the  conclusion  that  sex  is  unalterably  fixed 
at  the  time  the  egg  is  fertilized,  by  definite  deter- 
miners which  act  in  the  same  way  as  other  Mendelian 
determiners.  Dr.  Shull,  whose  exhaustive  studies  in 
sex  determination  place  him  in  the  front  rank  as  an 
authority  on  the  subject,  makes  this  conservative 
statement:  "Nearly  all  the  recent  investigations 
indicate  that  sex  is  at  least  predominantly  dependent 
upon  the  genotypic  nature  of  the  individual." 

If  this  is  so,  while  it  furnishes  the  best  of  confirma- 
tory evidence  in  support  of  Mendel's  law,  it  shows 
that  it  is  not  possible  for  man  to  predetermine  the 
sex  of  his  offspring,  which  he  has  long  hoped  to  be 
able  to  do.  The  following  quotation  from  Castle 
may  suitably  close  this  chapter:  "Negative  as  are 
the  results  of  our  study  of  sex  control,  they  are  perhaps 


THE   DETERMINATION   OF   SEX  223 

not  wholly  without  practical  value.  It  is  something 
to  know  our  limitations.  We  may  thus  save  time 
from  useless  attempts  at  controlling  what  is  un- 
controllable and  devote  it  to  more  profitable  employ- 
ments." 


CHAPTER  XI 

THE   APPLICATION    TO    MAN 
1.  The  Application  of  Genetics  to  Man 

Human  civilization  goes  hand  in  hand  with  the 
degree  of  successful  interference  which  man  exerts 
upon  the  natural  forces  surrounding  him. 

Primitive  man  was  overwhelmed  and  outmastered 
by  his  environment,  but  civilized  man  harnesses  nature 
to  do  his  will.  Savages  are  not  proficient  in  the 
arts  of  cultivating  plants  and  domesticating  animals, 
while  these  are  the  very  things  upon  which  human  prog- 
ress fundamentally  depends.  The  degree  of  civiliza- 
tion of  any  people  is  closely  correlated  with  the  degree 
of  their  success  in  exercising  a  conquering  control 
over  plants  and  animals.  Any  knowledge  of  the 
laws  of  heredity,  therefore,  as  applied  by  man,  either 
directly  to  himself  or  indirectly  to  animals  and  plants, 
is  a  distinct  contribution  to  human  progress. 

In  1900  the  National  Association  of  British  and 
Irish  Millers,  as  Kellicott  points  out,  being  dissatis- 
fied with  the  quality  and  quantity  of  the  annual 
wheat  yield,  engaged  Professor  Biffen  to  apply  his 
knowledge  of  heredity  to  the  practical  problem  of 
improving  their  wheat  crop.  The  characters  desired 
were  a    short    full  head,  beardlessness,    high    gluten 

224 


THE   APPLICATION   TO   MAN  225 

content,  immunity  to  rust,  strong  supporting  straw, 
and  a  high  yield  per  acre.  In  the  short  time  that 
has  elapsed.  Professor  Biffen  has  succeeded  in  pro- 
ducing strains  of  wheat  that  combine  all  these  de- 
sirable characters  to  a  remarkable  degree. 

Such  an  immediate  result  would  not  have  been  pos- 
sible before  1900,  when  the  rediscovery  of  Mendel's 
law  revolutionized  man's  know^ledge  of  the  action  of 
heredity  in  nature. 

This  same  knowledge  which  has  made  possible  the 
improvement  of  wheat  may  be  applied  to  the  breed- 
ing of  man,  for  there  is  no  reasonable  doubt  that 
man  belongs  in  the  same  evolutionary  series  with  all 
other  animals,  as  Darwin  showed,  and  is  consequently 
subject  to  the  same  natural  laws  to  a  considerable 
degree. 

It  must  be  admitted  that  thus  far  in  the  progress 
of  civilization  more  attention  has  been  directed  to 
the  scientific  breeding  of  animals  and  plants,  little 
as  that  has  been,  than  to  the  scientific  breeding  of 
man.  Let  us  hope  that  the  future  will  have  a  dif- 
ferent story  to  tell  ! 

2.  Modifying  Factors  in  the   Case  of  Man 

There  are  certain  qualifying  factors  which  make 
the  problems  of  genetics  somewhat  different  in  the 
case  of  man  than  of  other  organisms. 

For  example,  mankind  has  come  to  be  partially 
exempt  from  some  of  the  natural  laws  that  affect 
other  organisms.  Thus  with  respect  to  the  w^orkings 
of  natural  selection  man  is  partially  under  "grace" 

Q 


226  GENETICS 

rather  than  "law."  Nature  no  longer  "selects" 
good  eyes  in  man  by  long,  patient,  and  devious 
processes  when  poor  eyes  are  made  good  almost  in- 
stantly by  a  visit  to  the  oculist.  She  has  long  since 
given  up  providing  natural  weapons  of  defense  for 
those  who  have  the  w^its  to  supply  themselves  more 
efficiently  with  artificial  means  of  self-preservation, 
and  she  no  longer  attempts  to  improve  the  natural 
powers  of  locomotion  of  those  who  are  able  to  tame 
a  horse  to  ride  upon,  or  who  build  steamships,  rail- 
roads, automobiles  and  aeroplanes,  thus  accom- 
plishing at  once  what  would  require  ages  at  least  to 
evolve. 

Neither  does  the  law  of  the  survival  of  the  fittest  in 
its  original  sense  apply  equally  to  man  and  to  other 
organisms.  Human  society  to-day  protects  its  unfit 
in  hospitals,  asylums,  and  through  various  philan- 
thropies, while  physicians  devote  themselves  to  the 
art  of  prolonging  life  beyond  the  period  of  usefulness. 

We  do  not  desire  these  results  of  our  modern  civili- 
zation to  be  otherwise,  but  the  fact  remains  that  some 
of  the  most  inflexible  and  universal  "natural  laws" 
are  ineffective  in  the  case  of  man,  and  it  is  profitable 
to  bear  this  in  mind  when  applying  the  laws  of  ge- 
netics to  man. 

The  laboratory  for  human  heredity  is  the  wide 
world,  but  it  is  obvious  that  the  experimental  method 
which  has  proven  so  effective  in  studying  the  heredity 
of  animals  and  plants  is  impracticable  in  the  case  of 
man.  The  consideration  of  human  heredity,  there- 
fore, must  always  be  largely  from  the  statistical  side, 


THE   APPLICATION  TO   MAN  227 

consisting  in  an  analysis  of  experiments  already  per- 
formed rather  than  in  initiating  new  experiments. 

Such  institutions  as  insane  asylums,  prisons, 
sanitariums,  and  homes  for  the  unfortunate  are 
excellent  foci  for  studying  certain  phases  of  human 
heredity,  because  they  are  simply  convenient  places 
where  the  results  of  similar  experiments  in  genetics 
have  been  brought  together. 

3.   Experiments  in  Human  Heredity 
a.    The  Jukes 

A  classic  example  of  an  experiment  in  human  hered- 
ity which  has  been  partially  analyzed  by  the  statisti- 
cal method  is  that  furnished  by  Dugdale  in  1877  in 
the  case  of  "Max  Jukes"  and  his  descendants.  It 
includes  over  one  thousand  individuals,  the  origin  of 
all  of  whom  has  been  traced  back  to  a  shiftless,  illit- 
erate, and  intemperate  backwoodsman  who  started 
his  experiment  in  heredity  in  western  New  York 
when  it  was  yet  an  unsettled  wilderness. 

In  1877  the  histories  of  540  of  this  man's  progeny 
were  known,  and  that  of  most  of  the  others  was 
partly  known.  About  one  third  of  this  degenerate 
strain  died  in  infancy,  310  individuals  were  paupers 
who  all  together  spent  a  total  of  2300  years  in  alms- 
houses, while  440  were  physical  wrecks.  In  addition 
to  this,  over  one  half  of  the  female  descendants  were 
prostitutes,  and  130  individuals  were  convicted  crim- 
inals, including  7  murderers.  Not  one  of  the  entire 
family  had  a  common   school   education,    although 


228  GENETICS 

the  children  of  other  families  in  the  same  region 
found  a  way  to  educational  advantages.  Only  20 
individuals  learned  a  trade  and  10  of  these  did  so  in 
state's  prison. 

It  is  estimated  that  up  to  1877  this  experiment  in 
human  breeding  had  cost  the  state  of  New  York 
over  a  million  and  a  quarter  dollars,  and  the  end  is 
by  no  means  yet  in  sight. 

b.    The  descendants  of  Jonathan  Edwards 

In  striking  contrast  to  the  case  of  Max  Jukes  is 
that  of  Jonathan  Edwards,  the  eminent  divine, 
whose  famous  progeny  Winship  describes  as  follows: 
"1394  of  his  descendants  were  identified  in  1900,  of 
whom  295  were  college  graduates ;  13  presidents  of 
our  greatest  colleges,  besides  many  principals  of 
other  important  educational  institutions ;  60  physi- 
cians, many  of  whom  were  eminent;  100  and  more 
clergymen,  missionaries,  or  theological  professors; 
75  were  officers  in  the  army  and  navy ;  60  were 
prominent  authors  and  writers,  by  whom  135  books 
of  merit  were  written  and  published  and  18  impor- 
tant periodicals  edited;  33  American  States  and 
several  foreign  countries  and  92  American  cities  and 
many  foreign  cities  have  profited  by  the  beneficent 
influence  of  their  eminent  activity ;  100  and  more 
were  lawyers,  of  whom  one  was  our  most  eminent 
professor  of  law;  30  were  judges;  80  held  public 
office,  of  whom  one  was  vice-president  of  the  United 
States ;  3  were  United  States  senators  ;  several  were 
governors,  Members  of  Congress,  framers  of  state 


THE   APPLICATION  TO   MAN  229 

constitutions,  mayors  of  cities,  and  ministers  to  for- 
eign courts;  one  was  president  of  the  Pacific  Mail 
Steamship  Company ;  15  railroads,  many  banks,  in- 
surance companies,  and  large  industrial  enterprises 
have  been  indebted  to  their  management.  Almost 
if  not  every  department  of  social  progress  and  of 
public  weal  has  felt  the  impulse  of  this  healthy, 
long-lived  family.  It  is  not  known  that  any  one  of 
them  was  ever  convicted  of  crime." 

c.    The  Kallikak  Family 

A  more  convincing  experiment  in  human  heredity 
than  the  foregoing,  since  it  concerns  the  descendants 
of  two  mothers  and  the  same  father,  is  furnished  by  the 
recently  published  history  of  the  "  Kallikak  "  family.^ 

During  Revolutionary  days,  the  first  Martin  Kalli- 
kak, —  the  name  is  fictitious,  —  who  was  descended 
from  a  long  line  of  good  English  ancestry,  took 
advantage  of  a  feeble-minded  girl.  The  result  of 
their  indulgence  was  a  feeble-minded  son  who  be- 
came the  progenitor  of  480  known  descendants  of 
whom  143  were  distinctly  feeble-minded,  while  most 
of  the  others  fell  below  mediocrity  without  a  single 
instance  of  exceptional  ability. 

"  After  the  Revolutionary  war,  Martin  married  a 
Quaker  girl  of  good  ancestry  and  settled  down  to 
live  a  respectable  life  after  the  traditions  of  his 
forefathers.  From  this  legal  union  with  a  normal 
woman  there  have  been  496  descendants.      All  of 

1 "  The  Kallikak  Family."     H.  H.  Goddard.     The  Macmillan  Co. 


230  GENETICS 

these  except  two  have  been  of  normal  mentality  and 
these  two  were  not  feeble-minded.  .  .  .  The  fact 
that  the  descendants  of  both  the  normal  and  the 
feeble-minded  mother  have  been  traced  and  studied 
in  every  conceivable  environment,  and  that  the  re- 
spective strains  have  always  been  true  to  type,  tends 
to  confirm  the  belief  that  heredity  has  been  the 
determining  factor  in  the  formation  of  their  respec- 
tive characters." 

4.   Moral  and  Mental  Characters  behave 

LIKE  Physical  Ones 

These  instances  of  human  breeding  show  unmis- 
takably that  "blood  counts"  in  human  inheritance, 
even  though  the  hereditary  unit  characters  that  lead 
to  these  general  results  have  not  yet  been  analyzed 
with  the  clearness  that  is  possible  in  dealing  with  the 
characters  of  some  animals  and  plants. 

There  is  of  course  no  question  of  moral  and  mental 
traits  in  plants,  and  the  role  that  these  play  in  animals 
is  not  easy  to  determine ;  but  in  man  the  case  is 
undoubtedly  much  more  important  and  complex, 
since  mental  and  moral  characteristics  have  a  large 
share  in  making  man  what  he  is.  There  is,  however, 
no  fundamental  scientific  distinction  which  can  be 
drawn  between  moral,  mental,  and  physical  traits, 
and  they  are  undoubtedly  all  equally  subject  to  the 
laws  of  heredity. 

For  instance,  as  an  illustration  of  the  heritability 


THE   APPLICATION   TO   MAN  231 

of  non-physical  traits,  in  the  Jukes  pedigree  three 
of  the  daughters  of  Max  impressed  their  pecuh'ar 
moral  and  mental  characteristics  in  a  distinctive 
way  upon  their  offspring.  To  quote  Davenport  : 
"Thus  in  the  same  environment,  the  descendants  of 
the  illegitimate  son  of  Ada  are  prevailingly  criminal ; 
the  progeny  of  Belle  are  sexually  immoral;  and  the 
offspring  of  Effie  are  paupers.  The  difference  in  the 
germplasm  determines  the  difference  in  the  prevailing 
trait." 

5.   The  Character  of  Human  Traits 

Of  the  mental,  moral,  and  physical  traits  which 
are  heritable  in  man,  some  must  be  regarded  as 
generally  desirable,  some  as  indifferent,  and  others 
as  defects  to  be  avoided  if  possible.  In  general  the 
majority  of  human  traits,  those  which  together  make 
up  man  as  distinguished  from  other  animals,  do  not 
particularly  claim  the  attention  because  they  are  so 
universal.  Some  which  stand  out  from  the  mass, 
such  as  the  physical  traits  of  eye-color  and  the  color 
and  character  of  hair,  may  be  regarded  as  indifferent 
so  far  as  the  welfare  of  the  individual  is  concerned, 
while  others  like  skin  color  and  certain  racial  features 
that  characterize  particular  strains  of  *' blood"  may, 
under  certain  circumstances,  work  a  social  handicap 
upon  their  possessors  according  to  the  traditions  of 
the  community  in  which  they  appear. 

A  long  list  of  desirable  mental  traits  miglit  be 
enumerated  that  seem  in  a  general  way  to  be  subject 
to  the  laws  of  inheritance,,  although  they  have  not 


232  GENETICS 

yet  undergone  the  careful  analysis  demanded  by 
modern  genetics  which  deals  in  unit  characters  rather 
than  in  lump  inheritance. 

Musical,  literary,  or  artistic  ability,  for  example, 
mathematical  aptitude  and  inventive  genius,  as  well 
as  a  cheerful  disposition  or  a  strong  moral  sense  are 
probably  all  gifts  that  come  in  the  germplasm. 

They  may  each  be  developed  by  exercise  or  re- 
pressed by  want  of  opportunity,  nevertheless  they 
are  fundamentally  germinal  gifts. 

A  genius  must  be  born  of  potential  germplasm. 
No  amount  of  faithful  plodding  application  can  com- 
pensate for  a  lack  of  the  divine  hereditary  spark  at 
the  start. 

6.   Hereditary  Defects 

Undesirable  hereditary  traits  are  usually  defects 
due  to  the  absence  of  some  character.  For  instance, 
albinism,  which  occurs  in  several  kinds  of  animals 
and  also  in  man  in  one  out  of  every  20,000  individuals 
(according  to  Elderton),  is  due  to  the  absence  of  pig- 
ment in  the  skin,  hair  and  eyes.  Albinic  individuals 
have  poor  eyesight  because  they  are  unable  to  stand 
strong  light,  being  without  protective  pigment  in  the 
eyes.  This  peculiarity  of  albinism  behaves  as  a 
recessive  character  both  in  man  and  in  other  animals. 
An  albinic  individual  may,  therefore,  marry  a  normal 
individual  without  fear  of  producing  albino  children, 
although  the  children  of  such  a  mating  would  carry 
heterozygous  germplasm  with   respect   to   albinism, 


THE  APPLICATION   TO   MAN  233 

and  in  cousin  marriages  might  subsequently  produce 
some  albino  children. 

Davenport,  in  his  recent  work  on  "Heredity  in  Re- 
lation to  Eugenics,"  brings  together  a  long  catalogue 
of  human  hereditary  defects,  although  in  most 
instances  they  are  extremely  difficult  of  accurate 
analysis.  This  is  the  case,  first,  because  these  defects 
so  often  probably  depend  upon  a  combination  of 
determiners  rather  than  upon  a  single  one,  and,  sec- 
ond, because  the  available  data  are  usually  scattered 
and  incomplete. 

Deafness,  for  example,  is  a  defect  which  is  heredi- 
tary though  exactly  to  what  degree,  it  is  at  present 
impossible  to  state.  The  following  table  taken  from 
the  extensive  work  of  Fay  (1898)  upon  "Marriage  of 
the  Deaf  in  America"  gives  some  idea  of  the  results 
of  different  matings  lumped  together  statistically. 


Condition  op  Parents 

Percentage  of  Deaf 
Offspring 

Both  born  deaf 

25.9 

One  born  deaf,  one  wnth  acquired  deafness 

6.3 

One  bom  deaf,  one  normal        

11.9 

Both  with  acquired  deafness 

2.3 

One  with  acquired  deafness,  one  normal      .     . 

2.2 

That  two  parents  born  deaf  do  not  produce  more 
than  26  per  cent  of  deaf  children  is  probably  due  to 
the  fact,  first,  that  each  parent  is  in  all  likelihood  heter- 
ozygous for  deafness  and  that,  second,  the  same  com- 


234  GENETICS 

bination  of  factors  which  is  the  cause  of  the  parental 
defect  on  either  side  of  the  pedigree  does  not  happen  to 
recombine  after  segregation  to  form  the  new  individ- 
ual. Deafness  will  be  produced  in  the  offspring  only 
when  matings  occur  in  which  the  proper  factors  are 
combined.  Such  an  undesirable  result  is  much  more 
likely  to  happen  if  both  parents  come  from  the 
same,  or  related,  hereditary  strains  than  if  they  are 
derived  from  families  in  no  way  connected  by  blood. 

Herein  lies  the  biological  objection  to  cousin 
marriage  which  tends  to  bring  together,  and  thus 
to  perpetuate,  like  defects.  Outcrossing,  on  the 
contrary,  through  the  law  of  dominance,  tends  to 
conceal  defects  and  to  prevent  their  expression. 

Many  other  cases  of  human  defects,  such  as  im- 
becility or  insanity,  are  extremely  difficult  of  analysis 
from  the  standpoint  of  heredity  because,  in  the  first 
place,  the  defective  conditions  descriptively  included 
under  these  vague  terms  are  made  up  of  a  multitude 
of  diverse  conditions  each  of  which  must  have  a 
different  array  of  determiners  and,  in  the  second 
plaice,  because  any  one  definite  sort  of  insanity  or 
imbecility  may  be  conditioned  by  a  variety  of 
factors. 

However,  the  difficulty  of  the  problem  is  no 
reason  for  abandoning  the  attempt  to  reach  its  solu- 
tion and  to  learn,  if  possible,  "whence  come  our 
300,000  insane  and  feeble-minded,  our  160,000  blind 
or  deaf,  the  2,000,000  that  are  annually  cared  for  by 
our  hospitals  and  Homes,  our  80,000  prisoners  and 
the  thousands  of  criminals  that  are  not  in  prison, 


THE  APPLICATION   TO   MAN 


^285 


and  our  100,000  paupers   in   almshouses   and  out  " 
(Davenport) . 

7.   The  Control  of  Defects 

The  method  of  possible  control  of  human  defects 
depends  upon  whether  they  are  positive  or  negative, 
that  is,  dominant  or  recessive.  In  those  cases  where 
a  given  defect  is  due  to  a  single  determiner  the 
Mendelian  expectation  for  the  possible  offspring 
arising  from  various  matings  is  indicated  in  the  fol- 
lowing table  in  which  D  stands  for  the  defect  and  d 
for  its  absence :  — 


The  Mendelian  Expectation  for  Defects 


1 
2 
3 

4 
5 

6 

7 
8 

If  the  Defect  is  Positive 
(dominant) 

If  the  Defect  is  Negative 
(recessive) 

When    both 

DD  XDD  =  all  DD 

parents  show 

DD  XDd  =  hDD  +hDd 

dd  Xdd  =  all  dd 

the  defect 

DdXDd  =  \DD  +  hDd+\dd 

When    one 
parent  only- 

DD  Xdd  =  all  Dd 

dd  XDD  =  all  Dd 

shows     the 
defect 

DdXdd  =  hDd  +  hdd 

dd  XDd  =-  \Dd  +  \dd 

When     neither 

dd  Xdd  =  all  dd 

DD  XDD  =  all  DD 

parent  shows 

Dd  XDD  =  iDD  +  iDd 

the  defect 

DdXDd  =  i  DD  +i  Dd  +  i  dd 

If  the  defect  is  positive  and  in  a  duplex  or  homo- 
zygous condition  in  one  parent,  as  in  1,  2,  and  4 
above,  all  the  offspring  will  possess  it  regardless  of 
the  germinal  constitution  of  the  other  parent.     In 


236  GENETICS 

two  cases  only,  namely,  in  3  and  5,  where  the  de- 
fective parent  is  heterozygous,  is  there  any  chance 
of  unaffected  offspring,  and  even  in  these  cases  the 
defect  is  quite  as  likely  to  appear  as  not.  It  is  ob- 
vious that  the  only  way  to  rid  germplasm  of  a 
dominant  defect  is  by  continued  mating  with 
recessive  individuals.  By  this  method  it  is  possible 
in  time  to  shake  off  the  defect.  When  it  once  dis- 
appears in  any  individual,  it  will  never  return  unless 
crossed  back  to  a  similar  defective  dominant  strain. 

In  other  words,  such  a  recessive  extracted  from  a 
heterozygous  ancestry  will  breed  just  as  true  as  a 
recessive  which  was  pure  from  the  start.  In  both 
instances  there  is  an  entire  absence  of  the  character 
in  question,  and  it  is  clear  that  this  character  can 
thereafter  never  again  reappear,  since  something 
cannot  be  derived  from  nothing. 

On  the  other  hand,  if  a  defect  is  negative,  depending 
upon  the  absence  of  a  normal  dominant  determiner, 
as  is  usually  the  case  with  defects,  it  behaves  as  a 
Mendelian  recessive,  that  is,  it  is  always  apparent  in 
individuals  developing  from  the  homozygously  de- 
fective germplasm. 

It  is  certain,  for  example,  that  an  imbecile  which 
has  arisen  from  homozygous  defective  germplasm 
carries  only  the  determiner  for  imbecility  in  his  own 
germplasm,  and  when  two  such  recessives  mate,  noth- 
ing but  imbecile  offspring  can  result,  for  recessives 
breed  true.     Nothing  plus  nothing  equals  nothing. 

An  illustration  of  this  principle  is  given  in  the  fol- 
lowing pedigree  (Fig.  72)  furnished  by  Goddard,  1910. 


THE   APPLICATION   TO   MAN 


237 


The  result  is  quite  different,  however,  when  one 
parent  only  shows  the  defect.  If  the  other  parent  is  a 
normal  homozygote,  as  in  case  4  of  the  accompanying 
table,  all  the  offspring  will  be  normal  in  appearance, 
but  with  the  bar  sinister  of  defectiveness  in  their 
germplasm,  while  if  the  other  parent  is  heterozygous 
(Case  5),  one  half  of  the  progeny  will  be  defective. 
Finally,  when  neither  parent  shows  defectiveness 


EkO 


N 


c 


N]-r{N)@[r]®LN 

c 


[Nh<N)[rU^^ 


Fig.  72.  —  Pedigree  chart  illustrating  the  law  that  two  defective  parents 
have  only  defective  offspring.  A,  alcoholic  ;  C,  criminalistic  ;  d,  died  ; 
F,  feeble-minded  ;  T,  tubercular.    After  Goddard. 


but  one  carries  the  defect  as  a  heterozygote  (Case  7), 
then  there  will  be  no  defective  children,  while  if 
both  parents  are  heterozygous  there  is  one  chance  in 
four  that  the  offspring  will  be  defective. 

As  a  matter  of  fact,  defectives  usually  mate  with 
defectives  for  the  simple  reason  that  normals  ordi- 
narily avoid  them,  so  it  comes  about  that  streams  of 
poor  germplasm  naturally  flowing  together  tend  to 
the  inbreeding  of  like  defects. 


238  GENETICS 

Davenport^  lays  down  the  following  general  eugenic 
rules  for  the  guidance  of  those  who  would  produce 
offspring  wisely  :  "If  the  negative  character  is,  as  in 
polydactylism  and  night-blindness,  the  normal  char- 
acter, the  normals  should  marry  normals,  and  they 
may  be  even  cousins.  If  the  negative  character  is 
abnormal,  as  imbecility  and  liability  to  respiratory  dis- 
eases, then  the  marriage  of  two  abnormals  means  prob- 
ably all  children  abnormal ;  the  marriage  of  two  nor- 
mals from  defective  strains  means  about  one  quarter  of 
the  children  abnormal ;  but  the  marriage  of  a  normal 
of  the  defective  strain  with  one  of  a  normal  strain  will 
probably  lead  to  strong  children.  The  worst  possible 
marriage  in  this  class  of  cases  is  that  of  cousins  from 
the  defective  strain,  especially  if  one  or  both  have 
the  defect.  In  a  word,  the  consanguineous  marriage 
of  persons  one  or  both  of  whom  have  the  same 
undesirable  defect,  is  highly  unfit,  and  the  mar- 
riage of  even  unrelated  persons  who  both  belong  to 
strains  containing  the  same  undesirable  defect  is  un- 
fit. Weakness  in  any  characteristic  must  be  mated 
with  strength  in  that  characteristic;  and  strength 
may  be  mated  with  weakness." 

8.   Inbreeding 

The  whole  matter  of  inbreeding  and  the  part  it 
plays  in  emphasizing  defects  has  received  a  fresh 
interpretation  in  the  light  of  Mendelism. 

There  is  a  widespread  popular  belief  that  inbreed- 
ing is  injurious  and  that  it  is  necessary  to  outcross 

^  Davenport.     Rep.  of  Amer.  Breeders'  Assoc,  Vol.  VI,  p.  431,  1910. 


THE   APPLICATION   TO   MAN  239 

in  order  to  maintain  the  vigor  and  avoid  the  defects 
of  any  Hne.  In  the  case  of  mankind,  consanguineous 
marriage  of  various  degrees  has  long  been  forbidden 
by  law  or  custom  in  many  races,  particularly  among 
the  Jews,  Mohammedans,  Indians  and  Romans.  On 
the  other  hand,  the  Persians,  Greeks,  Phoenicians  and 
Arabs  have  freely  practised  inbreeding,  while  one  of 
the  longest  of  known  human  pedigrees,  a  royal  line 
of  Egypt,  was  notorious  for  close  inbreeding,  even  to 
the  mating  of  brother  and  sister. 

There  has  been  a  greater  degree  of  inbreeding  in 
the  Puritan  stock  of  New  England  than  is  commonly 
realized.  David  Starr  Jordan  points  out  that  a 
child  of  to-day,  supposing  no  inbreeding  of  relatives 
had  occurred,  would  have  had  in  the  time  of  William 
the  Conqueror,  thirty  generations  ago,  8, 598, 094, oO'^ 
living  ancestors.  If  this  theoretical  supposition 
were  really  so,  it  would  seem  quite  possible  for  every 
New  Englander  to-day  to  have  had  at  least  one  an- 
cestral representative  who  won  glory  under  William. 

The  difference  between  the  unthinkable  number 
given  above  and  the  actual  number  of  probable 
ancestors  alive  thirty  generations  ago  emphasizes 
the  fact  that  inbreeding  must  have  occurred  freely. 

There  are,  indeed,  various  well-known  provisions 
in  nature  to  insure  inbreeding.  The  majority  of 
plants  are  probably  self-fertihzed  while  hermaphro- 
ditic animals,  which  sometimes  at  least  are  self- 
fertilized  particularly  among  the  lower  forms,  are 
very  common. 

Nature  has  secured,  on  the  other  hand,  often  by 


240  GENETICS 

elaborate  devices,  a  separation  of  the  sexes,  especially 
among  the  higher  organisms,  and  in  consequence 
there  has  arisen  an  unavoidable  necessity  of  out- 
crossing. The  intricate  adaptations  existing  between 
insects  and  flowers,  for  example,  seem  to  be  directed 
entirely  toward  insuring  outcrossing  among  plants. 

9.   Experiments  to  test  the  Effect  of  In- 
breeding 

Numerous  experiments  to  test  the  effect  of  in- 
breeding have  been  carried  out  upon  various  or- 
ganisms. 

Darwin,  for  instance,  planted  morning-glories, 
Ipomoea,  derived  from  the  same  stock  of  seeds,  in 
two  beds  which  were  laid  out  side  by  side,  that  is, 
in  an  environment  as  nearly  the  same  as  possible, 
but  with  half  of  the  beds  screened  from  insects 
which  usually  transfer  pollen  from  flower  to  flower. 
In  the  screened  half  where  all  insects  were  excluded 
the  flowers  were  of  necessity  self-fertilized,  while  in 
the  exposed  half  they  were  presumably  cross-pol- 
linated by  the  insects  which  had  free  access  to  them. 
The  seeds  produced  in  the  two  beds  were  kept  sep- 
arate and  the  experiment  was  continued  for  ten  years, 
so  that  at  the  end  of  that  time  two  lots  of  morning- 
glories,  one  self-fertilized  for  ten  generations  and  the 
other  presumably  cross-pollinated  for  the  same  length 
of  time,  were  obtained  for  comparison.  The  crite- 
rion Darwin  used  was  the  vigor  of  the  plants  as 
shown  by  the  length  of  the  vine.     He  found  that  the 


THE   APPLICATION  TO   MAN  241 

cross-pollinated  plants  were  to  the  self-polliiiated 
ones  as  100  to  53,  and  his  conclusion  was  conse- 
quently, that  cross-pollination  is  beneficial  and  self- 
pollination  is  detrimental. 

Ritzema-Bos  inbred  rats  for  twenty  generations. 
For  the  first  ten  generations  the  average  number  of 
young  per  litter  was  7.5,  while  for  the  last  ten  genera- 
tions it  fell  to  3.2. 

Weismann  inbred  mice  for  twenty-nine  genera- 
tions and  obtained  a  parallel  result.  For  the  first 
ten  generations  the  average  number  per  litter  was 
6.1,  for  the  second  ten  generations  5.6,  and  for  the 
last  nine  generations  4.2. 

Shull  found  in  growing  Indian  corn  that  loss  of 
vigor  results  from  continual  self-fertilization,  and 
many  breeders  have  had  similar  experiences  with 
other  plants  and  animals. 

On  the  other  hand,  in  the  case  of  the  pomace  fly, 
Drosophila,  Castle  inbred  brother  and  sister  for 
fifty-nine  generations  without  diminishing  the  fer- 
tility of  the  line.  No  arbitrary  law  with  respect  to 
the  effects  of  inbreeding  upon  vigor  and  fertility 
can  be  laid  down,  therefore,  which  will  apply  equally 
to  all  cases. 

10.   The  Influence  of  Proximity 

Inbreeding  is  often  the  result  of  proximity.  In- 
sular or  isolated  communities,  slums  in  cities,  where 
those  of  one  language  herd  together,  or  hovels  in 
the  backwoods,  where  degenerates  of  a  kind  are  kept 
in  intimate  association,  as  well  as  asylums  of  various 


242  GENETICS 

sorts  in  which  similar  defectives  are  promiscuously 
housed  under  the  same  roof,  are  all  potent  agencies 
to  insure  human  inbreeding. 

Similarly,  localities  which  have  been  devastated 
by  migrations  of  the  most  effective  blood,  as,  for 
example,  parts  of  Ireland  or  many  rural  villages 
in  New  England,  are  frequently  characterized  by  a 
population  showing  a  large  percentage  of  defective- 
ness. The  able-bodied  and  ambitious  go  forth  into 
the  world  to  seek  their  fortunes,  while  the  deficient 
in  body  or  spirit  are  left  behind  where,  under  the 
spell  of  proximity,  they  perpetuate  their  deficiencies. 

The  part  that  improved  transportation  has  played 
in  mixing  up  populations  and  in  counteracting  the 
effects  of  stagnation  on  human  heredity,  through  in- 
breeding under  the  inertia  of  proximity  is  very  great. 
Before  the  days  of  railroads,  cousin-marriages  were 
much  more  frequent  than  they  are  now. 

From  a  biological  point  of  view  there  is  something 
to  be  said  in  favor  of  the  rape  of  the  Sabines  in  the 
past  and  for  the  pursuit  of  American  heiresses  by 
European  nobility  to-day. 

11.   Inbreeding  in  the  Light  of  Mendelism 

Inbreeding  in  itself  may  not  necessarily  be  injuri- 
ous. The  consequence  of  inbreeding  as  shown  by 
the  working  of  Mendelian  laws  is  that  latent  or 
recessive  characters  tend  to  become  homozygous  and 
so  brought  to  the  surface,  while  outcrossing  brings 
about  the  formation  of  heterozygous  traits  which 


THE   APPLICATION   TO   MAN  243 

mask  recessive  characters  and  render  them  inefTec- 
tive. 

Cousin-marriages,  although  producing  a  high  per- 
centage of  defects,  do  not  necessarily  reproduce  un- 
desirable traits.  They  simply  bring  out  latent  or 
recessive  characters  for  the  reason  that  under  these 
conditions  defect  meets  defect  instead  of  the  opposite 
normal  condition  which  would  dominate  the  defect 
and  cause  it  not  to  appear. 

Since  a  recessive  trait  is  properly  regarded  as  the 
absence  of  a  positive  dominant  character,  it  more 
frequently  stands  for  an  undesirable  feature  than 
otherwise.  Thus  it  comes  about  that  inbreeding, 
by  combining  negative  features,  may  "produce"  a 
defective   strain. 

Outcrossing  always  increases  heterozygous  com- 
binations in  the  germplasm  and  covers  up  undesirable 
recessive  traits  through  the  introduction  of  addi- 
tional dominant  traits.  Inbreeding,  on  the  contrary, 
tends  to  simplify  the  germplasm,  that  is,  to  make  it 
more  homozygous,  and  so  to  bring  recessive  defects 
to  the  surface. 


CHAPTER  XII 

HUMAN    CONSERVATION 
1.   How  Mankind  may  be  Improved 

There  are  two  fundamental  ways  to  bring  about 
human  betterment,  namely,  by  improving  the  in- 
dividual and  by  improving  the  race.  The  first 
method  consists  in  making  the  best  of  whatever 
heritage  has  been  received  by  placing  the  individual  in 
the  most  favorable  environment  and  developing  his 
capacities  to  the  utmost*  through  education.  The 
second  method  consists  in  seeking  a  better  heritage 
with  which  to  begin  the  life  of  the  individual.  The 
first  method  is  immediate  and  urgent  for  the  present 
generation.  The  second  method  is  concerned  with 
ideals  for  the  future,  and  consequently  does  not  usu- 
ally present  so  strong  an  appeal  to  the  individual. 

The  first  is  the  method  of  euthenics,  or  the  science 
of  learning  to  live  well.  The  second  is  eugenics , 
which  Galton  defines  as  "the  science  of  being  well 
born." 

These  two  aspects  of  human  betterment,  however, 
are  inseparable.  Any  hereditary  characteristic  must 
be  regarded,  not  as  an  independent  entity,  but  as  a 
reaction  between  the  germplasm  and  its  eyivironment. 
The  biologist  who  disregards  the  fields  of  educational 
endeavor  and  environmental  influence,  is  equally  at 

244 


HUMAN   CONSERA  ATIOX  245 

fault  with  the  sociologist  who  fails  sufficiently  to  real- 
ize the  fundamental  importance  of  the  germplasm. 

Without  euthenic  opportunity  the  best  of  hei-i- 
tages  would  never  fully  come  to  its  own.  Without 
the  eugenic  foundation  the  best  opportunity  fails 
of  accomplishment.  The  euthenic  })oint  of  view, 
however,  must  not  distract  the  attention  now,  for 
the  present  chapter  is  particularly  concerned  with 
the  program  of  eugenics. 

2.   More  Facts  Needed 

Since  the  point  of  attack  in  human  heredity  must 
be  largely  statistical,  it  is  of  the  first  importance  to 
collect  more  facts.  Our  actual  knowledge  is  con- 
fused with  a  mass  of  tradition  and  opinion,  much  of 
which  rests  upon  questionable  foundations.  The 
great  present  need  is  to  learn  more  facts ;  to  sift  the 
truth  from  error  in  what  is  already  known ;  and  to 
reduce  all  these  data  to  workable  scientific  form. 
Much  progress  is  being  made  in  this  direction,  owing 
to  the  impetus  given  by  the  revival  of  Mendel's 
illuminating  work,  but  as  yet  the  science  of  eugenics 
is  in  its  infancy. 

The  most  systematic  and  effective  attempt  in  this 
country  to  collect  reliable  data  concerning  heredity 
in  man  has  been  initiated  b}^  the  Eugenics  Section 
of  the  American  Breeders'  Association  under  the 
secretaryship  of  Dr.  C.  B.  Davenport.  In  1910  the 
Eugenics  Record  OflSce,  with  a  staff  of  expert  field 
and  office  workers  and  an  adequate  equipmcMit  of 
fire-proof  vaults,  etc.,  for  the  preservation  of  records, 


246  GENETICS 

was  opened  at  Cold  Spring  Harbor,  Long  Island,  New 
York,  w4th  Mr.  H.  H.  Laughlin  as  superintendent. 
*'The  main  work  of  this  office  is  investigation  into 
the  laws  of  inheritance  of  traits  in  human  beings  and 
their  application  to  eugenics.  It  proffers  its  serv- 
ices free  of  charge  to  persons  seeking  advice  as  to 
the  consequences  of  proposed  marriage  matings.  In 
a  word,  it  is  devoted  to  the  advancement  of  the 
science  and  practice  of  eugenics."  The  publica- 
tion of  results  from  the  Eugenics  Record  Office  has 
already  been  begun. 

The  Volta  Bureau,  founded  about  twenty-five 
years  ago  in  Washington  by  Dr.  Alexander  Graham 
Bell,  is  collecting  data  with  reference  to  deafness 
and  has  now  systematically  arranged  particulars  con- 
cerning the  history  of  over  20,000  individuals.  In 
England,  also,  the  Galton  Laboratory  for  Eugenics, 
founded  in  1905,  is  systematically  collecting  facts 
about  human  pedigrees  and  publishing  the  results  in 
a  compendious  "  Treasury  of  Human  Inheritance." 

Besides  these  special  bureaus  of  investigation, 
innumerable  facts  about  the  inheritance  of  particular 
traits  are  being  incidentally  brought  together  and 
made  available  in  various  institutions  and  asylums 
throughout  the  world  which  are  immediately  con- 
cerned with  the  care  of  defectives  of  different  types. 
It  is  in  connection  with  such  institutions  for  defec- 
tives that  much  of  the  most  successful  "field  w^ork" 
of  the  Eugenics  Section  of  the  American  Breeders' 
Association  is  being  accomplished  in  the  United 
States. 


HUMAN   CONSERVATION  ^2^7 

3.   Further  Application  of  What  we  know 

Necessary 

Human  performance  always  lags  behind  liuinan 
knowledge.  Many  persons  who  are  fully  aware  of 
the  right  procedure  do  not  put  their  knowledge  into 
practice.  It  follows,  therefore,  that  any  program  of 
eugenics  which  does  not  grip  the  imagination  of  the 
common  people  in  such  a  way  as  to  become  an  effec- 
tive part  of  their  very  lives  is  bound  to  remain  largely 
an  academic  affair  for  Utopians  to  quarrel  and  theo- 
rize over. 

It  is  not  enough  to  collect  facts  and  w^ork  out  an 
analysis  and  interpretation  of  them,  for,  important 
as  this  preliminary  step  is,  it  must  be  followed  by  a 
convincing  campaign  of  education. 

The  lives  of  the  unborn  do  not  force  themselves 
upon  the  average  man  or  woman  with  the  same 
insistency  as  the  lives  already  begun.  In  the  midst 
of  the  overwhelming  demands  of  the  present,  the 
appeal  of  posterity  for  better  blood  is  vague  and 
remote.  If  every  individual  regarded  the  germ- 
plasm  he  carries  as  a  sacred  trust,  then  it  would  be 
the  part  of  an  awakened  eugenic  conscience  to  restrain 
that  germplasm  when  it  is  known  to  be  defective  or, 
when  it  is  not  defective,  to  hand  it  on  to  posterity 
with  at  least  as  much  foresight  as  is  exercised  in 
breeding  domestic  animals  and  cultivated  plants. 

The  eugenic  conscience  is  in  need  of  development, 
and  it  is  only  when  this  becomes  thoroughly  aroused 
in  the  rank  and  file  of  society  as  well  as  among  the 


248  GENETICS 

leaders,   that  a  permament  and  increasing  better- 
ment of  mankind  can  be  expected. 

4.   The  Restriction  of  Undesirable  Germ- 
plasm 

A  negative  way  to  bring  about  better  blood  in  the 
world  is  to  follow  the  clarion  call  of  Davenport  and 
"dry  up  the  streams  that  feed  the  torrent  of  de- 
fective and  degenerate  protoplasm."  This  may 
be  partially  accomplished,  at  least  in  America,  by 
employing  the  following  agencies :  control  of  immi- 
gration ;  more  discriminating  marriage  laws ;  a 
quickened  eugenic  sentiment ;  sexual  segregation  of 
defectives ;  and  finally,  drastic  measures  of  asexuali- 
zation or  sterilization  when  necessary. 

a.    Control  of  Immigration 

The  enforcement  of  immigration  laws  tends  to 
debar  from  the  United  States  not  only  many  unde- 
sirable individuals,  but  also  incidentally  to  keep  out 
much  potentially  bad  germplasm  that,  if  admitted, 
might  play  havoc  with  future  generations. 

For  example,  during  the  year  of  1908,  65  idiots, 
121  feeble-minded,  184  insane,  3741  paupers,  2900 
individuals  having  contagious  diseases,  53  tuber- 
culous individuals,  136  criminals,  and  124  prostitutes 
were  caught  in  the  sieve  at  Ellis  Island  alone  and 
turned  back  from  this  country  by  the  immigration 
officials.  These  7000  and  more  individuals  probably 
were  the  bearers  of  very  little  germplasm  that  we 
are  nationally  not  better  off  without. 


HUMAN   CONSER\\\TION  210 

Eugenically,  the  weak  point  in  tlie  present  applica- 
tion of  immigration  laws  is  that  criteria  for  exekision 
are  phenotypic  in  nature  rather  than  genotypic,  and 
consequently  much  bad  germplasm  comes  throu<(li 
our  gates  hidden  from  the  view  of  inspectors  because 
the  bearers  are  heterozygous,  wearing  a  cloak  of 
desirability  over  undesirable  traits. 

It  is  not  enough  to  lift  the  eyelid  of  a  prospective 
parent  of  American  citizens  to  discover  whether  he 
has  some  kind  of  an  eye-disease  or  to  count  the 
contents  of  his  purse  to  see  if  he  can  pay  his  own 
way.  The  official  ought  to  know  if  eye-disease  rinis 
in  the  immigrant's  family  and  whether  he  comes  from 
a  race  of  people  which,  through  chronic  shiftlessness 
or  lack  of  initiative,  have  always  carried  light  purses. 

In  selecting  horses  for  a  stock-farm  an  expert 
horseman  might  rely  to  a  considerable  extent  upon 
his  judgment  of  horseflesh  based  upon  inspection 
alone,  but  the  wise  breeder  does  more  than  take  the 
chances  of  an  ordinary  horse  trader.  He  wants  to 
be  assured  of  the  pedigree  of  his  prospective  stock. 
It  is  to  be  hoped  that  the  time  will  come  when  we, 
as  a  nation,  will  rise  above  the  hazardous  methods  of 
the  horse  trader  in  selecting  from  the  foreign  appli- 
cants who  knock  at  our  portals,  and  that  we  will 
exercise  a  more  fundamental  discrimination  than 
such  a  haphazard  method  affords,  by  demanding  a 
knowledge  of  the  germplasm  of  these  candidates  for 
citizenship,  as  displayed  in  their  pedigrees. 

This  may  possibly  be  accomplished  by  haviiiL,' 
trained  inspectors  located  abroad  in  the  connnuui- 


250  GENETICS 

ties  from  which  our  immigrants  come,  whose  duty 
it  shall  be  to  look  up  the  ancestry  of  prospective 
applicants  and  to  stamp  desirable  ones  with  approval. 
The  national  expense  of  such  a  program  of  genealogi- 
cal inspection  would  be  far  less  than  the  mainte- 
nance of  introduced  defectives,  in  fact  it  would 
greatly  decrease  the  number  of  defectives  in  the 
country.  At  the  present  time  this  country  is  spending 
over  one  hundred  million  dollars  a  year  on  defectives 
alone,  and  each  vear  sees  this  amount  increased. 

The  United  States  Department  of  Agriculture 
already  has  field  agents  scouring  every  land  for 
desirable  animals  and  plants  to  introduce  into  this 
country,  as  well  as  stringent  laws  to  prevent  the  im- 
portation of  dangerous  weeds,  parasites,  and  organ- 
isms of  various  kinds.  Is  the  inspection  and  super- 
vision of  human  blood  less  important  ? 

b.    More  Discriminating  Marriage  Laws 

Every  people,  including  even  the  more  primitive 
races,  make  customs  or  laws  that  tend  to  regulate 
marriage.  Of  these,  the  laws  which  relate  to  the 
eugenic  aspect  of  marriage  are  the  only  ones  that 
concern  us  in  this  connection.  *' Marriage,"  says 
Davenport,  "can  be  looked  at  from  many  points  of 
view.  In  novels  as  the  climax  of  human  courtship ; 
in  law  largely  as  two  lines  of  property  descent ;  in 
society,  as  fixing  a  certain  status ;  but  in  eugenics, 
which  considers  its  biological  aspect,  marriage  is  an 
experiment  in  breeding." 

Certain    of    the    United    States    have    laws    for- 


HUMAN   CONSERVATION  251 

bidding  the  marriage  of  epileptics,  the  insane,  liahit- 
ual  drunkards,  paupers,  idiots,  feeble-minded,  and 
those  afflicted  with  venereal  diseases.  It  would  be 
well  if  such  laws  were  not  only  more  uniform  and 
widespread,  but  also  more  rigidly  enforced. 

It  is  quite  true  that  marriage  laws  in  themselves 
do  not  necessarily  control  human  reproduction,  for 
illegitimacy  is  a  factor  that  must  always  be  reckoned 
with ;  nevertheless  such  laws  do  have  an  important 
influence  in  regulating  marriage  and  consequent 
reproduction. 

Marriage  laws  may,  however,  sometimes  bring 
about  a  deplorable  result  eugenically,  as  in  the  case 
of  forced  marriage  of  sexual  offenders  in  order  to 
legalize  the  offense  and  "save  the  woman's  honor.'* 
To  compel,  under  the  guise  of  legality,  two  defective 
streams  of  germplasm  to  combine  repeatedly  and 
thereby  result  in  defective  offspring  just  because 
the  unfortunate  event  happened  once  illegitimately, 
is  fundamentally  a  mistake.  Darwin  says  :  "Except 
in  the  case  of  man  himself  hardly  any  one  is  so 
ignorant  as  to  allow  his  worst  animals  to  breed." 

c.   An  Educated  Sentiment 

A  far  more  effective  means  of  restricting  bad 
germplasm  than  placing  elaborate  marriage  laws 
upon  our  statute-books  is  to  educate  public  sentiment 
and  to  foster  a  popular  eugenic  conscience,  in  the 
absence  of  which  the  safeguards  of  the  law  must 
forever  be  largely  without  avail. 

Such  a  sentiment   already   generally   exists  to  a 


252  GENETICS 

large  extent  with  respect  to  incest,  and  the  marriage 
of  persons  as  noticeably  defective  as  idiots  or  those 
afflicted  with  insanity,  and  also  in  America  with  re- 
spect to  miscegenation,  but  a  cautious  and  intelligent 
examination  of  the  more  obscure  defective  traits, 
exhibited  in  the  somatoplasms  of  the  various  mem- 
bers of  families  in  question,  is  largely  an  ideal  of  the 
future.  Under  existing  conditions  non-eugenic  con- 
siderations such  as  wealth,  social  position,  etc.,  often 
enter  into  the  preliminary  negotiations  of  a  marriage 
alliance,  but  an  equally  unromantic  caution  with 
reference  to  the  physical,  moral,  and  mental  charac- 
ters that  make  up  the  biological  heritage  of  con- 
tracting parties  is  less  usual. 

The  scientific  attitude  is  not  necessarily^  opposed 
to  the  romantic  way  of  looking  at  things.  Science 
is  simply  "organized  common  sense,"  and  romance, 
that  dispenses  with  this  balance-wheel,  although  it 
may  be  entertaining  and  always  exciting  at  first, 
is  sure  to  be  disappointing  in  the  end.  Marriages 
may  be  "made  in  heaven,"  but,  as  a  matter  of  fact, 
children  are  born  and  have  to  be  brought  up  on  earth. 
It  follows  w^ithout  saying  that  it  will  be  much  easier 
to  stamp  out  bad  germplasm  when  an  educated  senti-> 
ment  becomes  common  among  all  people  everywhere. 

d.    Segregation  of  Defectives 

Persons  with  hereditary  defects,  such  as  epileptics, 
idiots,  and  certain  criminals,  who  become  wards  of 
the  state,  should  be  segregated  so  that  their  germ- 
plasm  may  not  escape  to  furnish  additional  burdens 


HUMAN   CONSER^'ATIOx\  258 

to  society.  "We  have  become  so  used  to  crime, 
disease  and  degeneracy  that  we  take  them  for  nec- 
essary evils.  That  they  were  in  the  worhFs  igno- 
rance, is  granted.  That  they  must  remain  so,  is 
denied ' '   (Davenport) . 

"The  great  horde  of  defectives  once  in  the  worhl 
have  the  right  to  Kve  and  enjoy  as  best  they  miiy 
whatever  freedom  is  compatible  with  the  lives  and 
freedom  of  other  members  of  society,"  says  KelHcott, 
but  society  has  a  right  to  protect  itself  against  repe- 
titions of  hereditarv  blunders. 

There  is  one  grave  danger  connected  willi  the 
administration  of  our  humane  and  commenihible 
philanthropies  toward  the  unfortunate,  for  it  fre- 
quently happens  that  defectives  are  kept  in  insti- 
tutions until  they  are  sexually  mature  or  are  partly 
self-supporting,  when  they  are  liberated  only  to  add 
to  the  burden  of  society  by  reproducing  their  like. 

Furthermore,  if  defectives  of  the  sanre  sort  are 
collected  together  in  the  same  institutions,  unless 
sexual  segregation  is  strictly  maintained,  they  may 
by  the  very  circumstance  of  proximity  tend  to  re- 
produce their  kind  just  as  defectives  in  any  isolated 
community  tend  to  multiply. 

David  Starr  Jordan  cites  the  interesting  case  of 
cretinism  which  occurs  in  the  valley  of  Aosta  in 
northern  Italy,  to  prove  the  wisdom  of  the  sexual 
segregation  of  defectives.  Cretinism  is  an  hereditary 
defect  connected  with  an  abnormal  develo})ment  of 
the  thyroid  gland  which  results  in  a  peculiar  form  of 
idiocy  usually  associated  with  goitre. 


254  GENETICS 

"In  the  city  of  Aosta  the  goitrous  cretin  has  been 
for  centuries  an  object  of  charity.  The  idiot  has 
received  generous  support,  while  the  poor  farmer  or 
laborer  with  brains  and  no  goitre  has  had  the  sever- 
est of  struggles.  In  the  competition  of  life  a  pre- 
mium has  thus  been  placed  on  imbecility  and  disease. 
The  cretin  has  mated  with  cretin,  the  goitre  with 
goitre,  and  charity  and  religion  have  presided  over 
the  union.  The  result  is  that  idiocy  is  multiplied 
and  intensified.  The  cretin  of  Aosta  has  been  de- 
veloped as  a  new  species  of  man.  In  fair  weather 
the  roads  about  the  city  are  lined  with  these  awful 
paupers  —  human  beings  with  less  intelligence  than 
a  goose,  with  less  decency  than  the  pig." 

Why mper,  writing  in  1880,  further  observes:  "It 
is  strange  that  self-interest  does  not  lead  the  natives 
of  Aosta  to  place  their  cretins  under  such  restrictions 
as  would  prevent  their  illicit  intercourse ;  and  it  is 
still  more  surprising  to  find  the  Catholic  Church 
actually  legalizing  their  marriage.  There  is  some- 
thing horribly  grotesque  in  the  idea  of  solemnizing 
the  union  of  a  brace  of  idiots,  and,  since  it  is  well 
known  that  the  disease  is  hereditary  and  develops  in 
successive  generations  the  fact  that  such  marriages 
are  sanctioned  is  scandalous  and  infamous." 

Since  1890  the  cretins  have  been  sexually  segregated, 

and  in  1910  Jordan  reported  that  they  were  nearly 

all  gone. 

e.    Drastic  Measures 

A  fifth  method  of  restricting  undesirable  germ- 
plasm   in   the   case   of   confirmed   criminals,    idiots. 


HUMAN   CONSERVATION  255 

imbeciles,  and  rapists  may  be  mentioned,  namely, 
the  extreme  treatment  of  either  asexuahzation  or 
vasectomy.  The  latter  is  a  minor  operation  con- 
fined to  the  male  which  occupies  only  a  few  moments 
and  requires  at  most  only  the  application  of  a  local 
ansesthetic,  such  as  cocaine.  There  are  no  disturbing 
or  even  inconvenient  after  effects  from  this  operation. 
It  consists  in  removing  a  small  section  of  each  sperm 
duct  and  is  entirely  effectual  in  preventing  subse- 
quent parenthood. 

In  the  female  the  corresponding  operation,  which 
consists  in  removing  a  portion  of  each  Fallopian 
tube,  is  much  more  severe,  but  not  impracticable  or 
dangerous. 

Eight  states  ^  already  have  sterilization  laws  pro- 
viding for  certain  cases  and  "could  such  a  law  be 
enforced  in  the  whole  United  States,  less  than  four 
generations  would  eliminate  nine  tenths  of  the  crime, 
insanity  and  sickness  of  the  present  generation  in 
our  land.  Asylums,  prisons  and  hospitals  would 
decrease,  and  the  problems  of  the  unemployed,  the 
indigent  old,  and  the  hopelessly  degenerate  would 
cease  to  trouble  civilization." 

5.   The  Conservation  of  Desirable  Germ  plasm 

Not  only  negatively  by  the  restriction  of  undesir- 
able germplasm,  but  also  positively  by  the  conser- 
vation of  desirable  germplasm,  may  the  eugenic 
ideal  be  approached. 

1  Indiana,  1907;  Washington,  1909;  California,  ^909;  Connecticut, 
1909;  Nevada,  1911 ;  Iowa,  1911  ;  New  Jersey,  1911  ;  New  York,  19U. 


256  GENETICS 

It  is  possible  that  if  some  of  the  philanthropic 
endeavor  now  directed  toward  alleviating  the  con- 
dition of  the  unfit  should  be  directed  to  enlarging  the 
opportunity  of  the  fit,  greater  good  would  result  in 
the  end.  In  breeding  animals  and  plants  the  most 
notable  advances  have  been  made  by  isolating  and 
developing  the  best,  rather  than  by  attempting  to 
raise  the  standard  of  mediocrity  through  the  elimina- 
tion of  the  worst. 

One  leader  is  worth  a  score  of  followers  in  any 
community,  and  the  science  of  genetics  surely  gives 
to  educators  the  hint  that  it  is  wiser  to  cultivate  the 
exceptional  pupil  who  is  often  left  to  take  care  of 
himself  than  to  expend  all  the  energies  of  the  in- 
structor in  forcing  the  indifferent  or  ordinary  one 
up  to  a  passing  standard.  The  campaign  for  human 
betterment  in  the  long  run  must  do  more  than  avoid 
mistakes.  It  must  become  aggressive  and  take  ad- 
vantage of  those  human  mutations  or  combinations 
of  traits  which  appear  in  the  exceptionally  endowed. 

There  are  various  ways  in  which  this  improvement 
of  society  may  be  brought  about. 

a.    By  Subsidizing  the  Fit 

The  following  unconfirmed  newspaper  clipping 
illustrates  the  point  of  what  is  meant  by  subsidizing 
the  fit  so  far  as  certain  physical  characteristics  are 
concerned.  "Berlin.  Dec.  11,  1911.  The  Empe- 
ror is  reported  to  be  interested  in  a  plan  proposed 
by  Professor  Otto  Hauser  for  the  propagation  of  a 
fixed   German   type   of   humanity,  —  a   type   which 


HUMAN   CONSERVATION  257 

will  be  as  fixed  as  the  Jewish  in  its  characteristics, 
if  the  suggestions  of  the  professor  can  ever  be  carried 
out.  The  fixed  type  is  to  be  produced  as  follows  :  — 
Only  'typical'  couples  are  to  be  allowed  to  mate. 
The  man  is  to  be  not  more  than  thirty  years  old, 
the  woman  not  over  twenty-eight,  and  each  have  a 
perfect  health  certificate.  The  man  should  be  at 
least  five  feet  seven  inches  tall ;  the  woman  not  under 
five  feet  six  inches.  Neither  the  man  nor  the  wonuin 
should  have  dark  hair.  Its  tint  mav  rani>'e  from 
blonde  to  auburn.  The  eyes  of  the  pair  should  be 
pure  blue  without  any  tint  of  brown.  The  complex- 
ion should  be  fair  to  ruddy  without  any  suggestion  of 
heaviness  or  'beefiness.'  The  nose  ought  to  be 
strong  and  narrow,  the  chin  square  and  powerful,  and 
the  skull  well  developed  at  the  back.  The  man  and 
the  woman  must  be  of  German  descent  and  must 
bear  a  German  name  and  speak  the  language  of 
Germany.  These  'mated  couples'  are  to  get  a 
wedding  gift  of  $125  and  an  additional  grant  for 
each  child  born.  The  couples  may  settle  in  the 
United  States  if  they  prefer."  This  reported  at- 
tempt to  establish  a  Prussian  type  of  "Hauser 
blondes"  at  least  points  the  way  to  one  sort  of  a 
positive  eugenic  method  that  might  possibly  be 
employed  with  respect  to  certain  physical  charac- 
teristics. 

It  should  be  remembered,  however,  that  the 
eugenic  ideal  is  not  by  any  means  confined  to  phys- 
ical traits  alone. 


s 


258  GENETICS 

b.    By  Eiilarging  Individual  Opportunity 

Much  good  human  germplasm  goes  to  waste 
through  ineffectiveness  on  account  of  unfavorable 
environment  or  lack  of  a  suitable  opportunity  to 
develop. 

Every  agency  which  contributes  toward  increasing 
the  opportunity  of  the  individual  to  attain  to  a 
better  development  of  his  latent  possibilities  is  in 
harmony  with  a  thoroughly  positive  eugenic  prac- 
tice. Thus  better  schools,  better  homes,  better 
living  conditions,  in  short,  all  euthenic  endeavor, 
directly  serves  the  eugenic  ideal  by  making  the  best 
out  of  whatever  germinal  equipment  is  present  in 
man. 

c.   By  Preventing  Germinal  Waste 

Much  good  protoplasm  fails  to  find  expression  in 
the  form  of  offspring  because  one  or  the  other  of 
possible  parents  is  cut  off  either  by  preventable 
death  or  by  social  hindrances.  To  avoid  such  ca- 
lamities is  a  part  of  the  positive  program  of  eugenics. 

1.   Preventable  Death 

War,  from  the  eugenic  point  of  view,  is  the  height 
of  folly,  since  presumably  the  brave  and  the  phys- 
ically fit  march  away  to  fight,  while  in  general  the 
unqualified  stay  at  home  to  reproduce  the  next  gen- 
eration. When  a  soldier  dies  on  the  battlefield  or  in 
the  hospital,  it  is  not  alone  a  brave  man  who  is  cut 
off,  but  it  is  the  termination  of  a  probably  desirable 
strain  of    germplasm.     The  Thirty  Years'    War    in 


HUMAN   CONSERVATION  259 

Germany  cost  6,000,000  lives,  while  Napoleon  in  his 
campaigns  drained  the  best  blood  of  France. 

David  Starr  Jordan  has  presented  this  matter 
very  clearly.  He  points  out  that  the  "man  with  a 
hoe"  among  the  European  peasantry  is  not  the 
result  of  centuries  of  oppression,  as  he  has  been 
pictured,  but  rather  the  dull  progeny  resulting  from 
generations  of  the  unfit  who  were  left  behind  when 
the  fit  went  off  to  war  never  to  return. 

Benjamin  Franklin,  with  characteristic  wisdom, 
sums  up  the  situation  in  the  following  epigram:  "Wars 
are  not  paid  for  in  war  time ;   the  bill  comes  later." 

2.    Social  Hindrances 

There  are  many  conditions  of  modern  society 
which  act  non-eugenically. 

For  instance,  the  increasing  demands  of  profes- 
sional life  prolong  the  period  necessary  for  prepara- 
tion, which,  with  the  "cost  of  high  living,"  tends 
toward  late  marriage.  In  this  way  much  of  the  best 
germplasm  is  very  often  withheld  from  circulation 
until  it  is  too  late  to  be  effective  in  providing  for 
the  succeeding  generation. 

Certain  occupations  such  as  school-teaching  and 
nursing  by  women  are  filled  by  the  best  blood  ob- 
tainable, yet  this  blood  is  denied  a  direct  part  in 
molding  posterity,  since  marriage  is  either  forbidden 
or  regarded  as  a  serious  handicap  in  such  lines  of 
work.  Advertisements  concerning  "unincumbered 
help"  and  "childless  apartments"  tell  their  own 
deplorable  tale. 


260  GENETICS 

One  of  the  darkest  features  of  the  dark  ages  from 
a  eugenic  standpoint  was  the  enforced  ceHbacy  of 
the  priesthood,  since  this  resulted,  as  a  rule,  in  with- 
drawing into  monasteries  and  nunneries  much  of 
the  best  blood  of  the  times,  and  this  uneugenic  cus- 
tom still  obtains  in  many  quarters  to-day. 

6.   Who  shall  sit  in  Judgment  ? 

In  the  practical  application  of  a  program  of  eu- 
genics there  are  many  difficulties,  for  who  is  quali- 
fied to  sit  in  judgment  and  separate  the  fit  from  the 
unfit  ? 

There  are  certain  strongly  marked  characteristics 
in  mankind  which  are  plainly  good  or  bad,  but  the 
principle  of  the  independence  of  unit  characters 
demonstrates  that  no  person  is  wholly  good  or  wholly 
bad.  Shall  we  then  throw  away  the  whole  bundle 
of  sticks  because  it  contains  a  few  poor  or  crooked 
ones  ? 

The  list  of  weakling  babies,  for  instance,  who  were 
apparently  physically  unfit  and  hardly  worth  raising 
upon  first  judgment,  but  who  afterwards  became 
powerful  factors  in  the  world's  progress,  is  a  notable 
one  and  includes  the  names  of  Calvin,  Newton, 
Heine,  Voltaire,  Herbert  Spencer  and  Robert  Louis 
Stevenson. 

Or,  take  another  example.  Elizabeth  Tuttle,  the 
grandmother  of  Jonathan  Edwards  whose  remarkable 
progeny  was  referred  to  in  a  preceding  chapter,  is 
described  as  "a  woman  of  great  beauty,  of  tall  and 
commanding  appearance,  striking  carriage,  of  strong 


HUMAN   CONSERVATION  ^2G1 

will,  extreme  intellectual  vigor  and  mental  grasp 
akin  to  rapacity,"  but  with  an  extraordinary  deficieunj 
in  moral  sense.  She  was  divorced  from  lier  hus- 
band "on  the  ground  of  adultery  and  other  immo- 
ralities. .  .  .  The  evil  trait  was  in  the  blood,  for  one 
of  her  sisters  murdered  her  own  son  and  a  brother 
murdered  his  own  sister."  That  Jonathan  Edwards 
owed  his  remarkable  qualities  largely  to  his  grand- 
mother rather  than  to  his  grandfather  is  shown  by 
the  fact  that  Richard  Edwards,  the  grandfather, 
married  again  after  his  divorce  and  had  five  sons  and 
one  daughter,  but  none  of  their  numerous  progeny 
"rose  above  mediocrity,  and  their  descendants  gained 
no  abiding  reputation."  As  shown  by  subsequent 
events,  it  would  have  been  a  great  eugenic  mistake 
to  have  deprived  the  world  of  Elizabeth  Tuttle's 
germplasm,  although  it  would  have  been  easy  to 
find  judges  to  condemn  her. 

Dr.  C.  V.  Chapin  recently  said  with  reference  to 
the  eugenic  regulation  of  marriage  by  physician's 
certificate:  "The  causes  of  heredity  are  many  and 
very  conflicting.  The  subject  is  a  difficult  one,  and 
I  for  one  would  hesitate  to  say,  in  a  great  many  cases 
where  I  have  a  pretty  good  knowledge  of  the  family, 
where  marriage  would,  or  would  not,  be  desirable." 

Desirability  and  undesirability  must  always  be 
regarded  as  relative  terms  more  or  less  indefinable. 
In  attempting  to  define  them,  it  makes  a  great  dif- 
ference whether  the  interested  party  holds  to  a  puri- 
tan or  a  cavalier  standard.  To  show  how  far  human 
judgment  may  err  as  well  as  how  radically  human 


262  GENETICS 

opinion  changes,  there  were  in  England,  as  recently  as 
1819,  233  crimes  punishable  by  death  according  to 
law. 

One  needs  only  to  recall  the  days  of  the  Spanish 
Inquisition  or  of  the  Salem  witchcraft  persecution  to 
realize  what  fearful  blunders  human  judgment  is 
capable  of,  but  it  is  unlikely  that  the  world  will  ever 
see  another  great  religious  inquisition,  or  that  in 
applying  to  man  the  newly  found  laws  of  heredity 
there  will  ever  be  undertaken  an  equally  deplorable 
eugenic  inquisition. 

It  is  quite  apparent,  finally,  that  although  great 
caution  and  broadness  of  vision  must  be  exercised  in 
bringing  about  the  fulfilment  of  the  highest  eugenic 
ideals,  nevertheless  in  this  direction  lies  the  future 
path  of  human  achievement. 


BIBLIOGRAPHY 

A  few  recent  works  of  a  general  nature  are  listed  below. 
Several  of  these  books  contain  bibliographies  of  technical  papers 
and  other  original  sources  of  information. 

For  the  past  five  years  The  American  Naturalist  has  been 
particularly  devoted  to  papers  on  the  problems  of  heredity. 

Bateson,  W.  1909.  Mendel's  Laws  of  Heredity.  Cambridge 
University  Press.     Cambridge. 

Baur,  E.  1911.  Einfuhrung  indie  experimentelleVererbungs- 
lehre.     Gebriider  Borntraeger.     Berlin. 

Castle,  W.  E.  1911.  Heredity  in  Relation  to  Evolution  and 
Animal  Breeding.     D.  Appleton  and  Co.     New  York. 

Coulter,  Castle,  Davenport,  East,  and  Tower.  1912. 
Heredity  and  Eugenics.  University  of  Chicago  Press. 
Chicago. 

Davenport,  C.  B.  1911.  Heredity'  in  Relation  to  Eugenics. 
Henry  Holt  and  Co.     New  York. 

Galton,  F.  1889.  Natural  Inheritance.  Macmillan  and  Co. 
London. 

Goddard,  H.  H.  1912.  The  Kallikak  Family.  The  Mac- 
millan Co.     New  York. 

CoRRENS,  C.  1912.  Die  neuen  Vererbungsgesetze.  Gebriider 
Borntraeger.     Berlin. 

Darbyshire,  a.  D.  1912.  Breeding  and  the  Mendelian  Dis- 
coverv.     Cassell  and  Co.,  Ltd.     London. 

East,  E.  M.  1907.  The  Relation  of  Certain  Biological  Prin- 
ciples  to    Plant    Breeding.     Bull.    158.     Conn.    Agric. 

Exp.   Sta. 

263 


264  BIBLIOGRAPHY 

GoDLEWSKi,  E.      1909.     Das  Vererbungsproblem  im  Lichte  der 

Entwicklungsmechanik.     Engelmann.     Leipzig. 
GoLDSCHMiDT,  R.      1911.     Einfuhrung  in  die  Vererbungswissen- 

schaft.     Engelmann.     Leipzig. 
JoHANSSEN,  W.      1909.     Elemente  der  exakten  Erblichkeitslehre. 

Fischer.     Jena. 
Haecker,    V.     1912.      Allgemeine    Vererbungslehre.      Vieweg 

und  Sohn.      Braunschweig. 
Kellicott,  W.  E.     1911.     The    Social  Direction  of   Human 

Evolution.     D.  Appleton  and  Co.     New  York. 
Lock,  R.  H.      1909.     Recent  Progress  in  the  Study  of  Variation, 

Heredity  and  Evolution.     Murray.     London. 
LoTSY,    J.    P.      1906-1908.       Vorlesungen    iiber    Descendenz- 

theorien.     Fischer.     Jena. 
Morgan,  T.  H.      1907.      Experimental   Zoology.     The  Mac- 

millan  Co.     New  York. 
Montgomery,  T.  H.     1906.     The  Analysis  of  Racial  Descent 

in  Animals.     Henry  Holt  and  Co.     New  York. 
PuNNETT,   R.    C.     1911.      Mendelism.     The   Macmillan    Co. 

New  York. 
Reid,  Archdall,      1905.     The  Principles  of  Heredity.     Chap- 
man and  Hall.     London. 
Thomson,  J.  A.      1908.     Heredity.     Murray.     London. 
Watson,  J.  A.  S.     1912.     Heredity.     The  Dodge  Publishing 

Co.     New  York. 
Weismann,  a.      1904.     The  Evolution  Theory.     London. 
ScHALLMAYER,  W.      1910.     Vcrcrbung  und  Auslese  im  Lebens- 

lauf  der  Volker.     Fischer.     Jena. 
DE  Vries,  H.     1905.     Species  and  Varieties,  their  Origin  by 

Mutation.     The  Open  Court  Publishing  Co.     Chicago. 


INDEX 


Abnormal  fertilization,  30. 
Abraxas,  216. 
Acquired  callosities,  91. 
Acquired    characters,    definition  of, 

78. 
Ageniapsis,  202. 
Agouti,  163,  171. 
Albinism,  59,  151,  232. 
Alcoholism,  93. 
Alternative  inheritance,  121. 
Ambly stoma,  90,  129. 
American  Breeders'  Assoc,  245. 
Amitosis,  20. 
Ammonites,  71. 
Amphimixis,  55,  81. 
Anaphase,  20. 
Ancon  sheep,  68. 
Andalusian  fowl,  175,  180. 
Angora  hair,  129. 
Antirrhinum,  173. 
Ants,  210. 
Aphids,  204. 

Appendix,  vermiform,  150. 
Arithmetical  mean,  45. 
Armadillo,  202. 
Arrested  development,  149. 
Artistic  ability,  232. 
Ascaris,  11,  17. 
Asexualization,  255. 
Asexual  spores,  9. 
Atavism,  146. 
Autosomes,  209. 
Average  deviation,  45. 
Axolotl,  90. 
Azaleas,  double,  63. 


Balls,  129. 

Baltzer,  209,  220. 

Banana  fly  (see  Drosophila). 

Banded  shell,  129. 

Barley,  129,  154. 

Barrier  of  Linnaeus,  62. 


Bateson,  2,  41,  55,  69,   124,    129. 

130,   133,    149,    160,    161.    171. 

173,  180,  182. 
Battle  scars,  87. 
Baur,  129,  130,  173,  180. 
Beans,  102,  115. 
Beardless  barley,  129. 
Beech  leaves,  43,  49. 
Beech,  purple,  63. 
Beecher,  70. 
Bees,  210. 
Begonia,  7. 
Belemnites,  71. 
Belgian  hare,  183,  193. 
Bell,  246. 

Beneden,  van,  21,  23. 
Bertillon  system  of  identification.  38. 
BiFFEN,  129,  224. 
Bimodal  polygons,  48. 
Biological  inheritance,  75. 
Biometry.  42. 
Birth-marks,  87. 

Blending  inheritance,  121,  174,  182. 
Blonde  type,  257. 
Blumenbach,  197. 
Boleyn,  Anne,  59. 
Booted  poultry,  182. 
Born,  200. 

BovERi,  11,  17,  18,  24,  25,  32.  207. 
Brachydactyly,  129. 
Branched  habit,  129. 
Brooks,  75. 
Bryonia,  222. 
Bryozoa,  8. 

BURBANK,  156. 


Calvin,  260. 
Canaries,  129. 
Capsella,  89. 
Captivity,  effect  of.  152. 
Carnations,  double,  63. 
Carriers  of  color-blindness.  215. 

265 


266 


INDEX 


Castle,  4, 59, 124, 141, 163, 166, 170, 
171, 183,  202-205,  210, 222,  241. 
Castration,  210. 
Cats,  albino,  59. 

tailless,  68. 
Cattle,  birth-rate  of,  201. 

hairless,  68. 

hornless,  129. 

roan,  176. 

tailless,  68. 
Celandine,  63. 
Cell,  germ,  21. 

polar,  23. 

resting,  18. 

theory,  14. 

typical,  15. 

wall,  16. 
Centrosome,  17. 
Cerebral  hernia,  69. 
Chapin,  261. 
Characters,  congenital,  80. 

moral  and  mental,  230. 

unit,  61. 
Chauvin,  Marie  von,  90. 
Cheerful  disposition,  232. 
Chelidonium,  63,  71. 
Chromatin,  16. 
Chromosomes,  16. 

extra,  207. 

"x,"  207. 

"y,"  209. 
Chromosome  theory,  29. 
Chrysanthemum,  50. 
Circumcision,  87. 
Cirripede,  212. 
Clover,  four-leaved,  39. 
Cobb,  173,  183. 
Cochins,  182. 
Cocoon,  yellow,  129. 
Colorado    potato-beetle    (see    Lep- 

tinotarsa) . 
Color-blindness,  214. 

factor,  170. 
Comb  characters,  69. 
Complementary  factor,  159. 
Compound  determiners,  159. 
Congenital  characters,  80. 
CONKLIN,  29,  88. 

Consanguineous  marriage,  238,  239. 
Constants,  44. 


Continuity  of  germplasm,  11. 
Convergent  variation,  150. 
CoRRENs,  124,  129,  133,  175,  205, 

222. 
Cotton,  129. 

Cousin  marriage,  234,  238,  242,  243. 
Crabs,  212. 
Crested  head,  69,  129. 
Cretinism,  253. 
CuENOT,  164,  170,  171,  200. 
Cumulative  factor,  159,  187. 
Curly  hair,  137. 
Currant-worm,  216. 
Cytoplasm,  15. 

Daisy,  50. 
Daphnids,  53,  204. 
Darbyshire,  128,  177. 
Darwin,  1,  23,  39,  40,  42,  52,  55, 
56,  61,  67,  73,  77,  85,  86,  114, 
118,    123,    124,    151,   204,   225, 
240,  251. 
Datura,  180. 

Davenp6rt,  46,  59,  68,   124,   129, 
148,   152,    173,   177,   178,   179, 
181,    182,  231,   233,   235,  237, 
245,  248,  250,  253. 
Deafness,  233. 
Death,  7. 

preventable,  258. 
Defects,  hereditary,  232. 
Delayed  dominance,  177. 
Department  of  Agriculture,  250. 
Dermal  spines,  151. 
Determiners  of  heredity,  28. 
Deviation,  average,  45. 

standard,  46. 
Dihybrids,  133. 
Dinosaurs,  71. 
Diluting  factor,  165,  171. 
Dioecious  plants,  220. 
Disease  transmission,  92. 
Docked  tails,  88. 
Dogs,  hairless,  68. 

tailless,  68. 

trimmed  ears  of,  88. 
Dominance,  145,  174. 

delayed,  177. 

imperfect,  175. 

reversed,  178. 


INDEX 


267 


Dominant,  125,  148,  158. 
DONCASTER,  217,  218. 
Double  flowers,  63. 
Dreylincourt,  197. 
Drinkard,  129. 
Drosophila,  129,  241. 

DUGDALE,  227. 

Duplex  dose,  133. 
Durham,  165,  171. 

East,  192. 

Echidna,  150. 

Echinoderms,  31. 

Echinus,  32. 

Education,  3. 

Edwards,  Jonathan,  228,  260. 

Richard,  261. 
Egg,  abortive,  23. 

cell,  21. 

fertilized,  24. 

human,  28. 

mature,  23. 
ElMER,  40. 
Elderton,  232. 
Elementary  species,  62,  72. 
Environment,  2,  88. 
Enzyme  theory,  33. 
Equality  of  sexes,  201. 
Erinaceus,  150. 
Erithizon,  150. 
Eugenic  regulation  of  marriage,  261. 

rules,  238. 

sentiment,  251. 
Eugenics,  244. 

Record  Office,  245. 
Euthenics,  244. 

Evening  primrose  (see  (Enothera) . 
Extension  factor,  166,  170. 
Extra  chromosome,  207. 
Extracted  recessive,  148. 
Extra  toe,  69,  178. 
Eye  color  in  man,  121,  147,  177,231. 

in  Drosophila,  129. 

Factor,  color,  170. 
complementary,  159. 
compound,  159. 
cumulative,  159,  187. 
diluting,  165,  171. 
extension,  166,  170. 


hypothesis,  150. 

inhibitory,  160. 

intensifying,  165,  171. 

pattern,  163,  171. 

pigment,  101. 

restriction.  160,  170. 

spotting,  171. 

supplementary,  159,  163. 

uniformity,  104,  171. 
False  reversion,  149. 
Farrabee,  129. 
Fay,  233. 
Feathers,  absent,  09. 

branched,  69. 

recurved,  69. 

twisted,  69. 
Feeble-mindedness,  149. 
Feet  of  Chinese  women,  87. 
Feral  animals,  150,  152. 
Fertilization,  24. 

abnormal,  30. 

selective,  202. 
Firefly,  207. 
Fission,  7. 
Flavism,  151. 
Fluctuation,  57. 
Four-leaved  clover,  39. 
Four-o'clock,  175,  180. 
Four-toed  guinea-pigs.  59. 
Franklin,  Benjamin,  259. 
Freaks,  58. 
Freckles,  81. 
Frogs,  200. 

G.A.GE,  83. 

Galen,  198. 

G.A.LTON,  42,  77,  98,  117.  120-122. 

151,  164,  244. 
Galton    Laboratory    for    Eugenics, 

246. 
Gamete,  23. 

Garden  peas,  124-128,  132.  134. 
Geddes,  197. 
Gemmules,  8. 

Generation,  spontaneous,  6. 
Genetics,  2. 
Genotype,  113,  148. 
Germ-cells,  21. 
Germplasm,  10,  14S. 
GoDDARD,  229,  236,  237. 


268 


INDEX 


GOLDSCHMIDT,  44. 

Graduated  variants,  108. 
Greyhounds,  33,  201. 
Greening  apple,  8. 
Green  peas,  134. 
Growth,  7. 
Gruber,  16. 
Guinea-pig,  agouti,  163. 

albino,  59. 

angora,  129. 

brown-eyed  yellow,  166. 

coat  character,  141. 

four-toed,  59. 

Haecker,  17,  129. 
Hair  color,  137,  177. 
Hairlessness,  68. 
Hallet,  152. 
Hare,  Belgian,  183,  193. 
Harelip,  149. 
Hauser,  256. 
Hedgehog,  European,  150. 
Heine,  260. 
Helix,  178. 
Henking,  207. 
Hereditary  bridge,  27. 
Heredity,  definition  of,  4. 
Hereford  cattle,  68. 
Heritage,  2. 

Hermaphrodites,  220,  239. 
Heterozygote,  131. 
Hippocrates,  198. 
His,  29. 
Hodge,  92. 
Homozygote,  131. 
HOOKE,  15. 
Hornless  cattle,  129. 
Horns  in  sheep,  179. 

in  cattle,  179. 
Horses,  birth-rate  of,  201. 

hairless,  68. 

pacing,  129. 

trotting,  129. 
Human  betterment,  244. 

birth-rate,  201. 

egg,  28. 

skin  color,  196. 
Human  stature,  98. 

traits,  231. 
Hurst,  133. 


Hyalodaphnia,  54. 
Hybridization,  120. 
Hymenoptera,  210. 

Illegitimacy,  251. 

Imbecility,  234,  238. 

Immigration,  248. 

Immunity  to  rust,  129. 

Imperfect  dominance,  175. 

Inachus,  212. 

Inbreeding,  238. 

Independent  unit  characters,  144. 

Indian  corn,  241. 

Induction,  parallel,  83,  93. 

somatic,  83. 
Influence  of  proximity,  241. 
Inheritance,  alternative,  121. 

biological,  75. 

blending,  121. 

particulate,  121,  164. 

sex-limited,  213. 
Inhibitory  factor,  181. 
Insanity,  234. 
Insects  and  flowers,  240. 
Instinct,  92. 

Intensifying  factor,  165,  171. 
Inventive  genius,  232. 
Ipomoea,  240. 

Japanese  art,  37. 
Jamestown  weed,  180. 
Jennings,  48,  103,  110,  120. 
JOHANSSEN,   43,  102,  110,  114,  115, 

119,  155. 
Jordan,  253,  254,  259. 
Jukes,  227,  231. 
Jungle-fowl,  148. 

Kallikak  family,  229. 
Kammerer,  90. 
Kellicott,  224,  253. 
Kelvin,  5. 
King,  200. 
Klebs,  54. 
kolliker,  von,  21. 

Lamarck,  53,  76. 
Lamarck's  evening  primrose,  64. 
Lang,  129,  178,  180,  185,  193. 
Lathyrus,  160. 


INDEX 


1^09 


Laughlin,  246. 

Lederbaur,  89. 

Leghorn  poultry,  177,  182. 

Legless  Iamb,  39. 

Leptinotarsa,  108. 

Liability  to  respiratory  diseases,  238. 

Linnaean  species,  60. 

Lint,  129. 

Literary  ability,  232. 

Live-for-ever,  54. 

LoEB,  27. 

Lop-eared  rabbit,  183. 

Lychnis,  67,  222. 

MacDougall,  67,  83. 

Maize,  129,  192. 

Mantle  fibers,  19. 

Many-celled  fruit,  129. 

Marriage,  consanguineous,  238,  239. 

cousin,  234,  238,  242,  243. 

laws,  250. 
Marschal,  202. 
Mathematical  aptitude,  232. 
Maturation,  22. 
McClung,  207. 
Mean,  arithmetical,  45. 
Melanism,  151. 
Mendel,  1,  123,  130,  132,  136,  140, 

143,  157,  159,  174,  202,  245. 
Mendel's  law,   125,   127,   144,   174, 

222,  225. 
Mental  characters,  230. 
Membrane,  nuclear,  15. 
Merino  sheep,  68. 
Metaphase,  19. 
Mice,  albino,  59. 

decaudalized,  88. 

hairless,  68. 

in  abnormal  temperature,  89. 

inbred,  241. 

intensified  color,  165. 

piebald,  121,  164. 

waltzing,  129. 

yellow,  203. 
Mid-parent,  99. 
Miescher,  33. 
Milton,  6. 
Mirabilis,  175,  180. 
Mitosis,  18. 
Mode,  45. 


MOHL,  VON,  15. 
Monochorial  twins,  201. 
Monoecious  plants,  220. 
Monohybrids,  131,  147. 
Montgomery,  34,  78,  80. 
Moral  characters,  230. 
Morgan,  Lloyd,  Mi. 
Morgan,  T.  H.,  129,  205,  208. 
Morning-glory,  240. 
Mud  puppy,  91. 
Mullenix,  173,  183. 
Musical  ability,  232. 
Mutation,  56. 

degressive,  59. 

progressive,  59. 

regressive,  59. 

theory,  56,  72. 
Mutilations,  87. 

Nageli,  38,  40,  123. 

Napoleon,  259. 

Nat.   Assoc,   of    British   and   Irish 

Millers,  224. 
Natural  selection,  118,  225. 
Neck  bare  in  poultry,  69. 
Necturus,  91. 
Neo-Darwinians,  77. 
Neo-Lamarckians,  77. 
Neo-Mendelian  theory  of  sex,  205. 
Nettles,  129. 
Newman,  202. 
Newton,  260. 
Nicodemus,  85. 
Night-blindness,  238. 
Nilsson-Ehle,  185. 

NiTOBE,  37. 

Nuclear  membrane,  15. 
Nucleus,  15. 
Nulliplex  dose,  133. 
Nutrition  theory  of  sex,  198. 

(Enothera,  64. 
Oesterleben,  201. 
Oil-gland,  69. 
Oocyte,  23,  211. 
Oogonia,  23. 
Ophiotrocha,  212. 
Origin  of  life,  5. 
of  species,  1. 
Ovid,  6. 


270 


INDEX 


Ovists,  22. 
Oyster-borer  snail,  47. 

Pacing  habit,  129. 

Palatine  ridges,  149. 

Pangenesis,  85,  114. 

Papaver,  63. 

Parallel  induction,  93. 

Paramecium,  48,  110. 

Parthenogenesis,  26,  107,  210. 

Partial  potency,  180. 

Particulate  inheritance,  121,  164. 

Pasteur,  6. 

Pattern  factor,  163,  171. 

Patterson,  202. 

Pearson,  42,  43,  49. 

Peas,  garden,   124,  132,   134,   177. 

sweet,  13,  160. 
Petunias,  double,  63. 
Pflijger,  200. 
Phacechcerus,  91. 
Phaseolus,  102. 
Phenotype,  113,  148. 
Phyrrhocoris,  207. 
Plesiosaurs,  71. 
Pigeons,  148,  201. 
Pigment  factor,  161. 
Pigs,  birth-rate  of,  201. 
Polar  cells,  23,  211. 
Polydactylism,  238. 
Pomace  fly  (see  Drosophila). 
Poppy,  Shirley,  63. 
Population,  115. 
Porcupine,  150. 
Potency,  defined,  179. 

failure  of,  180. 

partial,  180. 

total,  179. 
Poultry,  birth-rate  of,  201. 

booted,  182. 

tailless,  68,  129,  181. 

and  the  factor  hypothesis,  173. 

and  Sudan  Red  III,  83. 

and  white  color,  179. 
Predisposition  to  disease,  93. 
Prenatal  influences,  87. 
Presence   and   absence   hypothesis, 

132. 
Preventable  death,  258. 
Price,  129. 


Primitive  man,  224. 
Primroses,  double,  63. 

Lamarck's  evening,  64. 
Prophase,  18. 
Protophyta,  7. 
Protoplasm,  15. 
Protozoa,  7,  10. 
Proximity,  influence  of,  241. 
Pug  jaws,  69. 
PUNNETT,  127,  216. 
Pure  line,  103. 
Puritan  stock,  239. 
Purple  beech,  63. 

Quadruple  hybrids,  143. 
Quail,  92. 

Rabbits,  albino,  59. 

birth-rate  of,  201. 

color  in,  13,  169. 

ears  of,  183,  193. 

gray,  171. 

lop,  183. 
Radiolaria,  17. 
Rats,  albino,  59. 

inbred,  241. 
Raynor,  217,  218. 
Recessive,  126,  148,  157. 

extracted,  148. 
Redfield,  79. 
Red  flowers,  129. 
Regeneration,  210. 
Regression,  98,  151. 
Regressive  varieties,  72. 
Reid,  78. 
Reinfection,  94. 
Relative  variability,  47. 
Repair,  7. 

Reproduction,  sexual,  9,  20. 
Resting  cell,  18. 
Restriction  factor,  166,  170. 
Reversed  dominance,  178. 
Reversion,  146. 

false,  149. 
Riddle,  83. 
RiMPAU,  153. 
Ritzema-Bos,  241. 
Roan  color  of  cattle,  176. 
Rock-pigeon,  148. 
Roses,  double,  63. 


INDEX 


271 


Rotifers,  204. 
Rumplessness,  129,  181. 
Rust,  129. 

Sacculina,  212. 
Sadler,  198. 
Salamandra,  90. 
Saunders,  129. 
Scalp  muscles,  149. 
SCHENK,  198,  199. 
schleiden,  14. 
Schwann,  14. 

Sea-urchin,  32,  207,  209,  220. 
Secondary  sexual  characters,  210. 
Sedum,  54. 

Segregation  of  unit  characters,  130, 
145,  175. 

of  defectives,  252. 
Selective  fertilization,  202. 
Serrated  leaves,  129. 
Sex-limited  inheritance,  213. 
Sexual  reproduction,  9,  20. 
Sheep,  67,  179. 
Sheep,  Ancon,  68. 
Shirley  poppy,  63. 
Shull,  67,  129,  133,  222,  241. 
Siamese  twins,  69. 
Silk-worm,  129. 
Simplex  dose,  133. 
SiTOWSKI,  83. 
Skew  polygons,  50. 
Skin  color  in  man,  196,  231. 
Smith,  212. 

Smooth-margined  nettle  leaves,  129. 
Smooth  peas,  129,  134. 
Snails,  58,  129,-'178,  180. 
Snapdragon,  129,  173. 
Social  hindrances,  259. 
Somatic  induction,  83. 
Somatoplasm,  10,  148. 
Species,  62. 

cycle,  71. 

elementary,  62,  72. 

Linnaean,  60. 

origin  of,  1. 
Speculations  about  sex,  197. 
Spencer,  92,  260. 
Spermatocyte,  23,  211. 
Spermatogonia,  23. 
Spermatid,  23. 


Spermatozoa,  21,  23. 

Spermists,  22. 

Spha;r echinus,  32. 

Spines,  dermal,  151. 

Spiny  ant-eater,  150. 

Spillman,  129. 

Sponges,  7,  8. 

Spontaneous  generation,  G. 

Spores,  9. 

Sports,  39,  56. 

Spotting  factor,  171. 

Spotted  mice,  121,  164. 

Sprengel,  63. 

Spurs,  extra  in  poultry,  69. 

Standard  deviation,  46. 

Standfuss,  70. 

Starchy  kernel,  129. 

Starfish  rays,  44. 

Statoblasts,  8. 

Stentor,  16. 

Sterilization  laws,  255. 

Stevens,  207. 

Stevenson,  R.  L.,  260. 

Stockard,  39. 

Stocks,  double,  63. 

Straight  hair,  138. 

Sub-species,  62. 

Sudan  red  III,  83. 

Sugar  beet,  155. 

Sumner,  89. 

Sunburn,  88. 

Sunflower,  129. 

Supplementary  factors,  159,  163. 

Survival  of  the  fittest,  226. 

Susceptibility  to  disease,  93. 

to  rust,  129. 
Sweet  pea,  13,  160. 

Taillessness,  68. 
Tails,  docked,  88. 
Tatusia,  202. 
Telophase,  20. 
Tennent,  31. 
Theory,  cell,  14. 

chromosome,  29. 

enzyme,  33. 

mutation,  56,  72. 

nutrition,  198. 
Theromorphs,  71. 
Thigh  bones  absent,  69. 


272 


INDEX 


Thomson,  146,  197. 
Thumb  prints,  38. 
Thury,  198. 
Tineola,  83. 
Toes,  crippled,  87. 

extra,  69,  178. 

webbed,  69. 
Toenails,  absent,  69. 

extra,  69. 
Tomato,  129. 
Total  potency,  179. 
Tower,  52,  55,  108. 
TOYAMA,  129. 
Training,  2. 
Traits,  human,  231. 
Treasury    of    Human    Inheritance, 

246. 
Trees  deformed  by  wind,  88. 
Triangle  of  life,  1. 
Trihybrids,  140. 
TrUobites,  71. 
Trimmed  ears  of  dogs,  88. 
Trotting  habit,  129. 

TSCHERMAK,  VON,   124,   129. 

Tuberculosis,  38. 
Tuttle,  Elizabeth,  260. 
Twins,  201. 
Two-celled  fruit,  129. 
Tyndall,  6. 

Uniformity  factor,  164,  171. 
Unit  character,  61. 
Urosalpinx,  47. 

Variability,  relative,  47. 
Variation,  abnormal,  40. 

continuous,  40. 

convergent,  150. 

definite,  39. 

discontinuous,  41,  69. 

fluctuating,  43. 

fortuitous,  39. 

germinal,  40. 

graduated,  52. 

harmful,  39. 

hereditary,  41. 

indefinite,  39. 

indifferent,  39. 

integral,  52. 

morphological,  38. 

multiple,  39. 


non-hereditary,  41. 

normal,  40. 

orthogenetic,  39. 

physiological,  38. 

psychological,  38. 

quantitative,  41. 

qualitative,  41. 

single,  39. 

somatic,  40. 

useful,  39. 
Varieties,  62,  72. 
Vasectomy,  255. 
Vermiform  appendix,  150. 
Vestigial  structures,  149. 

ViLMORIN,  155. 

Viola,  37. 
Vitalism,  82. 
Volta  Bureau,  246. 
Voltaire,  260. 

Vries,  de,   56,   59,  62,  64,   67,  71, 
124,  129,  154,  155. 

Walter,  48,  173,  183. 

Waltzing  habit,  129. 

W^ar,  258. 

Wart-hog,  91. 

Warts  and  toads,  87. 

Wasps,  210. 

Weakling  babies,  260. 

Webbed  toes,  69. 

Weismann,     7,     10,     55,     77,     78, 

81,    84,    86-88,    94,     95,    123, 

241. 
Wheat,  129,  152,  186,  224. 
Whymper,  254. 
Wiedersheim,  87. 
Wilson,  34,  35,  207,  208. 
Wing  absent,  69. 
WiNSHiP,  228. 
Woltereck,  53. 
Wright,  Seth,  67. 
Wrinkled  kernel,  129,  134,  177. 

X  chromosome,  207. 

Y  chromosome,  209. 
Yellow  mice,  203. 

peas,  134. 
Yung,  200. 

Zygote,  24. 


fHOfERTT  UMAIF 


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kingdom.  It  differs  from  many  of  the  college  text-books  of  zcxilogy  now 
on  the  market  in  several  important  respects:  (i)  the  animals  and  their 
organs  are  not  only  described,  hut  their  functions  are  pointed  out;  (2)  the 
animals  described  are  in  most  cases  native  species;  and  (3)  the  relations 
of  the  animals  to  man  are  emphasized.  Besides  serving  as  a  text-book,  it 
is  believed  that  this  book  will  he  of  interest  to  the  general  reader,  since  it 
gives  a  bird's-eye  view  of  the  entire  animal  kingdom  as  we  know  it  at  the 
present  time. 

Within  the  past  decade  there  has  been  a  tendency  for  teachers  of  zoology 
to  pay  less  attention  to  morphology  and  more  to  physiology.  As  a  promi- 
nent morphologist  recently  said,  "  Morphology  ...  is  no  longer  in  favor 
.  .  .  and  among  a  section  of  the  zoological  world  has  almost  fallen  into 
disgrace"  (Bourne).  The  study  of  the  form  and  structure  of  animals  is, 
however,  of  fundamental  importance,  and  is  absolutely  necessary  before 
physiological  processes  can  be  fully  understood;  but  a  course  which  is 
built  up  on  the  "  old-fashioned  morphological  lines"  is  no  longer  adequate 
for  the  presentation  of  zoological  principles. 

The  present  volume  has  not  been  made  by  merely  adding  a  descripti'>n 
of  the  vertebrates  to  the  author's  "  Introduction  to  Zoology"  (for  a  brief 
account  of  which  see  the  last  pages  of  this  circular).  On  the  contrar)-.  it 
is  a  new  work  throughout,  although  the  same  general  method  of  treatment, 
which  proved  so  successful  in  the  earlier  book,  has  been  employed  in  this 
one.  Similarly,  in  the  preparation  of  this  book  the  author  has  submitted 
the  manuscript  of  each  chapter  to  a  scholar  and  teacher  of  un(|uestioned 
authority  in  the  particular  field.  The  criticisms  and  suggestions  thus  se- 
cured have  greatly  increased  both  the  accuracy  and  the  practicability  of 
the  text. 


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The  Cefl 
in  Development  and  Inheritance 

By 

EDMUND    B.   WILSON,   Ph.D. 

Professor  of  Zoology,  Columbia  University 

Second  Edition,  Revised  and  Enlarged 

Illustrated,  8vo,  $3.S0  net 

Since  the  appearance  of  the  first  edition  of  this  work,  in  1896,  the  aspect 
of  some  of  the  most  important  questions  with  which  it  deals  has  materially 
changed,  most  notably  in  case  of  those  that  are  focussed  in  the  centrosome 
and  involve  the  phenomena  of  cell-division  and  fertilization.  This  has 
necessitated  a  complete  revision  of  the  book,  many  sections  having  been 
entirely  rewritten,  while  minor  changes  have  been  made  on  almost  every 
page. 

It  has  therefore  been  considerably  enlarged,  and  upwards  of  fifty  new 
illustrations  have  been  added.  The  endeavor  has,  however,  still  been 
made  to  keep  the  book  within  moderate  limits,  even  at  some  cost  of  com- 
prehensiveness ;  and  the  present  edition  aims  no  more  than  did  the  first 
to  cover  the  whole  vast  field  of  cellular  biology.  Its  limitations  are,  as 
before,  especially  apparent  in  the  field  of  botanical  cytology.  Here  prog- 
ress has  been  so  rapid  that,  apart  from  the  difficulty  experienced  by  a 
zoologist  in  the  attempt  to  maintain  a  due  sense  of  proportion  in  review- 
ing the  subject,  an  adequate  treatment  would  have  required  a  separate 
volume.  The  author  has  therefore,  for  the  most  part,  considered  the 
cytology  of  plants  in  an  incidental  way,  endeavoring  only  to  bring  the 
more  important  phenomena  into  relation  with  those  more  fully  considered 
in  the  case  of  animals. 

The  steady  and  rapid  expansion  of  the  literature  of  the  general  subject 
renders  increasingly  difficult  the  historical  form  of  treatment  and  the  cita- 
tion of  specific  authority  in  matters  of  detail.  This  plan  has  nevertheless 
still  been  followed. 


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An  Outline  of 
the  Theory  of  Organic  Evolution 

With  a  Description  of  some  of 
the  Phenomena  'which  it  explains 

By  MAYNARD    M.    METCALF,    Ph.D. 

Professor  of  Biology  in  the  Woman's  College  of  Baltimore 

Second  Edition,  Revised 

Cloth,  8vo,  Colored  Plates,  $2.50  net 

The  lectures  out  of  which  this  book  has  grown  were  written  for  the 
author's  students  at  the  Woman's  College  of  Baltimore,  and  for  others  in 
the  college  not  familiar  with  biology  who  had  expressed  a  desire  to  attend 
such  a  course  of  lectures.  The  book  is,  therefore,  not  intended  for  biolo- 
gists, but  rather  for  those  who  would  like  a  brief  introductory  outline  of 
this  important  phase  of  biological  theory. 

It  has  been  the  author's  endeavor  to  avoid  technicality  so  far  as  possible, 
and  present  the  subject  in  a  way  that  will  be  intelligible  to  those  unfamiliar 
with  biological  phenomena.  The  subject,  however,  is  somewhat  intricate, 
and  cannot  be  presented  in  so  simple  a  manner  as  to  require  no  thought 
on  the  reader's  part;  but  it  is  hoped  that  the  interest  of  the  subject  will 
make  the  few  hours  spent  in  the  perusal  of  this  book  a  pleasure  rather 
than  a  burden. 

In  many  instances  matter  that  might  have  been  elaborated  in  the  text 
has  been  treated  in  the  pictures,  which,  with  their  appended  explanations, 
form  an  essential  part  of  the  presentation  of  the  subject.  This  method  of 
treatment  has  been  chosen  both  for  the  sake  of  the  greater  vividness  thus 
secured  and  because  it  enables  the  book  to  be  reduced  to  the  limits  de- 
sired. Many  of  the  illustrations  have  been  obtained  from  books  with 
which  the  reader  may  wish  later  to  become  familiar. 


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